U.S. patent application number 11/263626 was filed with the patent office on 2006-03-16 for laser gas replenishment method.
Invention is credited to Hans-Stephen Albrecht, Vadim Berger, Igor Bragin, Sergei Govorkov, Juergen Kleinschmidt, Thomas Schroeder, Uwe Stamm, Klaus Wolfgang Vogler, Wolfgang Zschocke.
Application Number | 20060056478 11/263626 |
Document ID | / |
Family ID | 33515030 |
Filed Date | 2006-03-16 |
United States Patent
Application |
20060056478 |
Kind Code |
A1 |
Albrecht; Hans-Stephen ; et
al. |
March 16, 2006 |
Laser gas replenishment method
Abstract
Output beam parameters of a gas discharge laser are stabilized
by maintaining a molecular fluorine component at a predetermined
partial pressure using a gas supply unit and a processor. The
molecular fluorine is subject to depletion within the discharge
chamber. Gas injections including molecular fluorine can increase
the partial pressure of molecular fluorine by a selected amount.
The injections can be performed at selected intervals to maintain
the constituent gas substantially at the initial partial pressure.
The amount per injection and/or the interval between injections can
be varied, based on factors such as driving voltage and a
calculated amount of molecular fluorine in the discharge chamber.
The driving voltage can be in one of multiple driving voltage
ranges that are adjusted based on system aging. Within each range,
gas injections and gas replacements can be performed based on, for
example, total applied electrical energy or time/pulse count.
Inventors: |
Albrecht; Hans-Stephen;
(Gottingen, DE) ; Vogler; Klaus Wolfgang;
(Eckental, DE) ; Kleinschmidt; Juergen;
(Weissenfels, DE) ; Schroeder; Thomas; (Gottingen,
DE) ; Bragin; Igor; (Gottingen, DE) ; Berger;
Vadim; (Gottingen, DE) ; Stamm; Uwe;
(Gottingen, DE) ; Zschocke; Wolfgang;
(Noerten-Hardenberg, DE) ; Govorkov; Sergei; (Boca
Raton, FL) |
Correspondence
Address: |
STALLMAN & POLLOCK LLP
353 SACRAMENTO STREET
SUITE 2200
SAN FRANCISCO
CA
94111
US
|
Family ID: |
33515030 |
Appl. No.: |
11/263626 |
Filed: |
October 31, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10338779 |
Jan 6, 2003 |
6965624 |
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11263626 |
Oct 31, 2005 |
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10114184 |
Apr 1, 2002 |
6504861 |
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10338779 |
Jan 6, 2003 |
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09734459 |
Dec 11, 2000 |
6389052 |
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10114184 |
Apr 1, 2002 |
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09447882 |
Nov 23, 1999 |
6490307 |
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10114184 |
Apr 1, 2002 |
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09780120 |
Feb 9, 2001 |
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10338779 |
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09738849 |
Dec 15, 2000 |
6678291 |
|
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10338779 |
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09453670 |
Dec 3, 1999 |
6466599 |
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10338779 |
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60171717 |
Dec 22, 1999 |
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60124785 |
Mar 17, 1999 |
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60182083 |
Feb 11, 2000 |
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60173993 |
Dec 30, 1999 |
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60170919 |
Dec 15, 1999 |
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60128227 |
Apr 7, 1999 |
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Current U.S.
Class: |
372/55 |
Current CPC
Class: |
G03F 7/70575 20130101;
H01S 3/036 20130101; H01S 3/02 20130101; H01S 3/08045 20130101;
H01S 3/08031 20130101; G02B 5/1838 20130101; H01S 3/1055 20130101;
H01S 3/134 20130101; H01S 3/0971 20130101; G03F 7/70025 20130101;
H01S 3/08009 20130101; H01S 3/08004 20130101; H01S 3/038 20130101;
G02B 5/1814 20130101; H01S 3/225 20130101 |
Class at
Publication: |
372/055 |
International
Class: |
H01S 3/22 20060101
H01S003/22 |
Claims
1. A gas discharge laser system, comprising: a discharge chamber
containing a laser gas mixture including a constituent gas which is
subject to depletion; a plurality of electrodes connected to a
power supply circuit for providing a driving voltage to said
electrodes as a pulsed discharge to energize said laser gas
mixture; a resonator surrounding said discharge chamber for
generating a pulsed laser beam; a gas supply unit connected to said
discharge chamber; and a processor for controlling gaseous flow
between said gas supply unit and said discharge chamber, the
processor configured to control the gas supply unit to inject a
selected amount of constituent gas into said discharge chamber at
selected intervals, at least one of the selected intervals and the
selected amount of each constituent gas injection depending upon an
amount of input electrical energy applied to the pulsed discharge
in the laser gas mixture; wherein the selected amount of the
constituent gas injection for at least one of the selected
intervals is between 0.0001 mbar and 0.2 mbar of said constituent
gas or between 0.003% and 7% of said constituent gas presently
within said discharge chamber.
2. A system according to claim 1, further comprising: a counter in
communication with the processor and operable to store an
accumulated total for the amount of input electrical energy applied
to the pulsed discharge.
3. A system according to claim 1, wherein: the processor is
configured to inject one of a plurality of selected amounts of
constituent gas at each interval, the timing between intervals
being further dependent upon the selected amount of constituent
gas.
4. A system according to claim 1, wherein: at least one of the
selected intervals and the amount of each constituent gas injection
is further dependent upon at least one parameter selected from the
group consisting of: the aging of at least one component of the
laser system, a calculated amount of the constituent gas in the gas
mixture after a previous injection, a measured pressure in an
accumulator from which constituent gas was previously injected, and
a measured temperature in one of said accumulator and said
discharge chamber.
5. A method for controlling a composition of a gas mixture within a
discharge chamber of a gas discharge laser system, comprising the
steps of: monitoring an amount of input electrical energy applied
to a pulsed discharge in the gas mixture; determining an amount of
constituent gas to be injected into said discharge chamber based on
the amount of input electrical energy applied to the pulsed
discharge, the amount of constituent gas to be injected for at
least one amount of input electrical energy being between 0.0001
mbar and 0.2 mbar of said constituent gas or between 0.003% and 7%
of said constituent gas presently within said discharge chamber;
selecting an interval at which to inject the constituent gas into
said discharge chamber, the interval being dependent upon the
amount of constituent gas; and injecting said amount of said
constituent gas into said discharge chamber at the selected
interval.
6. A method according to claim 5, further comprising: monitoring a
second parameter indicative of the concentration of a constituent
gas in the gas mixture.
7. A method according to claim 6, wherein: the second parameter is
selected from the group consisting of: the aging of at least one
component of the laser system, a calculated amount of the
constituent gas in the gas mixture after a previous injection, a
measured pressure in an accumulator from which constituent gas was
previously injected, and a measured temperature in one of said
accumulator and said discharge chamber
8. A method according to claim 7, wherein: at least one of the
amount of constituent gas to be injected and the interval at which
to inject the constituent gas is further determined using the
second parameter.
9. A gas discharge laser system, comprising: a discharge chamber
containing a laser gas mixture including a constituent gas which is
subject to depletion; a plurality of electrodes connected to a
power supply circuit for providing a driving voltage to said
electrodes as a series of pulsed discharges to energize said laser
gas mixture; a resonator surrounding said discharge chamber for
generating a pulsed laser beam; a gas supply unit connected to said
discharge chamber; and a processor for controlling gaseous flow
between said gas supply unit and said discharge chamber, the
processor configured to control the gas supply unit to inject an
amount of constituent gas into said discharge chamber at selected
intervals, at least one of the selected intervals and the amount of
each constituent gas injection depending upon the number of pulsed
discharges in the laser gas mixture, wherein the amount of the
constituent gas injection for at least one of the selected
intervals is between 0.0001 mbar and 0.2 mbar of said constituent
gas or between 0.003% and 7% of said constituent gas presently
within said discharge chamber.
10. A system according to claim 9, wherein: a counter in
communication with the processor and operable to store the number
of pulsed discharges in the laser gas mixture.
11. A system according to claim 9, wherein: the processor is
configured to inject one of a plurality of selected amounts of
constituent gas at each interval, the timing between intervals
being further dependent upon the selected amount of constituent
gas.
12. A system according to claim 9, wherein: at least one of the
selected intervals and the amount of each constituent gas injection
is further dependent upon at least one parameter selected from the
group consisting of: the aging of at least one component of the
laser system, a calculated amount of the constituent gas in the gas
mixture after a previous injection, a measured pressure in an
accumulator from which constituent gas was previously injected, and
a measured temperature in one of said accumulator and said
discharge chamber.
13. A method for controlling a composition of a gas mixture within
a discharge chamber of a gas discharge laser system, comprising the
steps of: monitoring a number of pulsed discharges in the gas
mixture; determining an amount of constituent gas to be injected
into said discharge chamber based on the number of pulsed
discharges; selecting an interval at which to inject the
constituent gas into said discharge chamber, the interval being
dependent upon the amount of constituent gas; and injecting said
amount of said constituent gas into said discharge chamber at the
selected interval, the amount of said constituent gas for at least
one selected interval being between 0.0001 mbar and 0.2 mbar of
said constituent gas or between 0.003% and 7% of said constituent
gas presently within said discharge chamber.
14. A method according to claim 13, wherein: monitoring a second
parameter indicative of the concentration of a constituent gas in
the gas mixture.
15. A method according to claim 14, wherein: the second parameter
is selected from the group consisting of: the aging of at least one
component of the laser system, a calculated amount of the
constituent gas in the gas mixture after a previous injection, a
measured pressure in an accumulator from which constituent gas was
previously injected, and a measured temperature in one of said
accumulator and said discharge chamber.
16. A method according to claim 15, wherein: at least one of the
amount of constituent gas to be injected and the interval at which
to inject the constituent gas is further determined using the
second parameter.
17. A gas discharge laser system, comprising: a discharge chamber
containing a laser gas mixture including a constituent gas which is
subject to depletion; a plurality of electrodes connected to a
power supply circuit for providing a driving voltage to said
electrodes as a pulsed discharge to energize said laser gas
mixture; a resonator surrounding said discharge chamber for
generating a pulsed laser beam; a gas supply unit connected to said
discharge chamber; and a processor for controlling gaseous flow
between said gas supply unit and said discharge chamber, the
processor configured to control the gas supply unit to inject an
amount of constituent gas into said discharge chamber at selected
intervals, at least one of the selected intervals and the amount of
each constituent gas injection depending upon an operation mode of
the gas discharge laser system, wherein the depletion rate of the
constituent gas in the laser gas mixture varies with operation
mode, and wherein the amount of constituent gas for at least one of
the selected intervals is between 0.0001 mbar and 0.2 mbar of said
constituent gas or between 0.003% and 7% of said constituent gas
presently within said discharge chamber
18. A system according to claim 17, wherein: the processor is
configured to inject one of a plurality of selected amounts of
constituent gas at each interval, the timing between intervals
being further dependent upon the selected amount of constituent
gas.
19. A system according to claim 17, wherein: at least one of the
selected intervals and the amount of each constituent gas injection
is further dependent upon at least one parameter selected from the
group consisting of: the aging of at least one component of the
laser system, a calculated amount of the constituent gas in the gas
mixture after a previous injection, a measured pressure in an
accumulator from which constituent gas was previously injected, and
a measured temperature in one of said accumulator and said
discharge chamber.
20. A method for controlling a composition of a gas mixture within
a discharge chamber of a gas discharge laser system, comprising the
steps of: monitoring an operation mode of the gas discharge laser
system, wherein a depletion rate of a constituent gas in the gas
mixture varies with operation mode; determining an amount of
constituent gas to be injected into said discharge chamber based on
the am operation mode of the gas discharge laser system; selecting
an interval at which to inject the amount of constituent gas into
said discharge chamber, the interval being dependent upon the
amount of constituent gas; and injecting said amount of said
constituent gas into said discharge chamber at the selected
interval, the amount of said constituent gas for at least one
selected interval being between 0.0001 mbar and 0.2 mbar of said
constituent gas or between 0.003% and 7% of said constituent gas
presently within said discharge chamber.
21. A method according to claim 20, wherein: monitoring a second
parameter indicative of the concentration of a constituent gas in
the gas mixture.
22. A method according to claim 20, wherein: the second parameter
is selected from the group consisting of: the aging of at least one
component of the laser system, a calculated amount of the
constituent gas in the gas mixture after a previous injection, a
measured pressure in an accumulator from which constituent gas was
previously injected, and a measured temperature in one of said
accumulator and said discharge chamber
23. A method according to claim 22, wherein: at least one of the
amount of constituent gas to be injected and the interval at which
to inject the constituent gas is further determined using the
second parameter.
24. A gas discharge laser system, comprising: a discharge chamber
containing a laser gas mixture including first and second
constituent gases, each of the first and second constituent gases
being subject to depletion; a plurality of electrodes connected to
a power supply circuit for providing a driving voltage to said
electrodes as a pulsed discharge to energize said laser gas
mixture; a resonator surrounding said discharge chamber for
generating a pulsed laser beam; a gas supply unit connected to said
discharge chamber; a processor for controlling gaseous flow between
said gas supply unit and said discharge chamber, the processor
configured to control the gas supply unit to inject a first amount
of the first constituent gas into said discharge chamber at one of
a first selected interval and to inject a second amount of the
second constituent gas into said discharge chamber at a second
selected interval; and a database in communication with the
processor and operable to store information about the first and
second constituent gases under different operating conditions of
the gas discharge laser system, whereby the processor can access
this information to determine values to be used for at least one of
the first amount and first interval and at least one of the second
amount and second interval at a present operating condition of the
laser system, wherein the first amount for at least one of the
first intervals is between 0.0001 mbar and 0.2 mbar of said first
constituent gas or between 0.003% and 7% of said first constituent
gas presently within said discharge chamber, and wherein the second
amount for at least one of the second intervals is between 0.0001
mbar and 0.2 mbar of said second constituent gas or between 0.003%
and 7% of said second constituent gas presently within said
discharge chamber.
25. A method for controlling a composition of a gas mixture within
a discharge chamber of a gas discharge laser system, comprising the
steps of: monitoring an operating condition of the gas discharge
laser system; accessing a database to determine a first amount of a
first constituent gas and a second amount of a second constituent
gas to be injected into said discharge chamber, the determined
first and second amounts being dependent upon the operating
condition of the gas discharge laser system; accessing the database
to determine a first interval at which to inject the first
constituent gas and a second interval at which to inject the second
constituent gas into said discharge chamber, the determined first
and second intervals being dependent upon the operating condition
of the gas discharge laser system; and injecting said first amount
of said first constituent gas and said second amount of said second
constituent gas into said discharge chamber at the respective first
and second intervals, wherein the first amount for at least one of
the first intervals is between 0.0001 mbar and 0.2 mbar of said
first constituent gas or between 0.003% and 7% of said first
constituent gas presently within said discharge chamber, and
wherein the second amount for at least one of the second intervals
is between 0.0001 mbar and 0.2 mbar of said second constituent gas
or between 0.003% and 7% of said second constituent gas presently
within said discharge chamber.
26. A method according to claim 25, wherein: monitoring a second
parameter indicative of the concentration of a constituent gas in
the gas mixture.
27. A method according to claim 25, wherein: the second parameter
is selected from the group consisting of: the aging of at least one
component of the laser system, a calculated amount of the
constituent gas in the gas mixture after a previous injection, a
measured pressure in an accumulator from which constituent gas was
previously injected, and a measured temperature in one of said
accumulator and said discharge chamber
28. A method according to claim 27, wherein: at least one of the
amount of constituent gas to be injected and the interval at which
to inject the constituent gas is further determined using the
second parameter.
Description
CLAIM OF PRIORITY
[0001] This application is a Continuation of U.S. application Ser.
No. 10/338,779, filed Jan. 6, 2003, which is a Continuation-in-Part
of U.S. application Ser. No. 10/114,184, filed Apr. 1, 2002, now
U.S. Pat. No. 6,504,861, which is a divisional of U.S. application
Ser. No. 09/734,459, filed Dec. 11, 2000, now U.S. Pat. No.
6,389,052, which claims the benefit of priority to U.S. provisional
patent application No. 60/171,717, filed Dec. 22, 1999, and which
is a Continuation-in-Part of U.S. application Ser. No. 09/447,882,
filed Nov. 23, 1999, which claims the benefit of U.S. provisional
application No. 60/124,785, filed Mar. 17, 1999. U.S. application
Ser. No. 10/338,779 also is a Continuation-in-Part of U.S.
application Ser. No. 09/780,120, filed Feb. 9, 2001, which claims
priority to U.S. provisional application No. 60/182,083, filed May
15, 2000. U.S. application Ser. No. 10/338,779 also is a
Continuation-in-Part of U.S. application Ser. No. 09/738,849, filed
Dec. 15, 2000, now U.S. Pat. No. 6,678,291, which claims priority
to U.S. provisional application Nos. 60/173,993, filed Dec. 30,
1999, and 60/170,919, filed Dec. 15, 1999, and is a
Continuation-in-Part of U.S. patent application Ser. No.
09/453,670, filed Dec. 3, 1999, now U.S. Pat. No. 6,466,599, which
claims priority to U.S. provisional application No. 60/128,227,
filed Apr. 7, 1999, and is a Continuation in Part of U.S. patent
application Ser. No. 09/599,130, filed Jun. 22, 2000, which claims
priority to U.S. provisional application No. 60/140,531, filed Jun.
23, 1999. U.S. application Ser. No. 10/338,779 also is a
Continuation-in-Part of U.S. application Ser. No. 09/826,301, filed
Apr. 3, 2001, now U.S. Pat. No. 6,556,609, which is a divisional of
U.S. application Ser. No. 09/453,670, filed Dec. 3, 1999, now U.S.
Pat. No. 6,466,599, which claims priority to U.S. provisional
application No. 60/128,227, filed Apr. 7, 1999. U.S. application
Ser. No. 10/338,779 also is a Continuation-in-Part of U.S.
application Ser. No. 10/077,328, filed Feb. 15, 2002, now U.S. Pat.
No. 6,546,037, which is a divisional of U.S. application Ser. No.
09/599,130, filed Jun. 22, 2000, now U.S. Pat. No. 6,381,256, which
claims priority to U.S. provisional application Nos. 60/140,531,
filed Jun. 23, 1999, and 60/204,095, filed May 15, 2000, and
60/162,735, filed Oct. 29, 1999, and 60/166,967, filed Nov. 23,
1999, and 60/170,342, filed Dec. 13, 1999, and which is a
Continuation-in-Part of U.S. application Ser. No. 09/317,527, filed
May 24, 1999, now U.S. Pat. No. 6,154,470, which claims priority to
U.S. provisional application Nos. 60/120,218, filed Feb. 12, 1999
and 60/119,486, filed Feb. 10, 1999. The above applications are
assigned to the same assignee as the present application and are
hereby incorporated herein by reference.
TECHNICAL FIELD OF THE INVENTION
[0002] The present invention relates to a method and apparatus for
stabilizing output beam parameters of a gas discharge laser. More
particularly, the present invention relates to maintaining an
optimal gas mixture composition over long, continuous operating or
static periods using very small gas injections.
BACKGROUND
[0003] Pulsed gas discharge lasers such as excimer and molecular
lasers emitting in the deep ultraviolet (DUV) or vacuum ultraviolet
(VUV) have become very important for industrial applications such
as photolithography. Such lasers generally include a discharge
chamber containing two or more gases such as a halogen and one or
two rare gases. KrF (248 .mu.m), ArF (193 nm), XeF (350 nm), KrCl
(222 nm), XeCl (308 .mu.m), and F.sub.2 (157 nm) lasers are
examples.
[0004] The efficiencies of excitation of the gas mixtures and
various parameters of the output beams of these lasers vary
sensitively with the compositions of their gas mixtures. An optimal
gas mixture composition for a KrF laser has preferred gas mixture
component ratios around 0.1% F.sub.2/1% Kr/98.9% Ne (see U.S. Pat.
No. 4,393,505, which is assigned to the same assignee and is hereby
incorporated by reference). A F.sub.2 laser may have a gas
component ratio around 0.1% F.sub.2/99.9% Ne or He or a combination
thereof (see U.S. Pat. No. 6,157,662, which is assigned to the same
assignee and is hereby incorporated by reference). Small amounts of
Xe may be added to rare gas halide gas mixtures, as well (see U.S.
patent application Ser. No. 09/513,025, which is assigned to the
same assignee and is hereby incorporated by reference; see also R.
S. Taylor and K. E. Leopold, Transmission Properties of Spark
Preionization Radiation in Rare-Gas Halide Laser Gas Mixes, IEEE
Journal of Quantum Electronics, pp. 2195-2207, vol. 31, no. 12
(December 1995). Any deviation from the optimum gas compositions of
these or other excimer or molecular lasers would typically result
in instabilities or reductions from optimal of one or more output
beam parameters such as beam energy, energy stability, temporal
pulse width, temporal coherence, spatial coherence, discharge
width, bandwidth, and long and short axial beam profiles and
divergences.
[0005] Especially important in this regard is the concentration (or
partial pressure) of the halogen, e.g., F.sub.2, in the gas
mixture. The depletion of the rare gases, e.g., Kr and Ne for a KrF
laser, is low in comparison to that for the F.sub.2. FIG. 1 shows
laser output efficiency versus fluorine concentration for a KrF
laser, showing a decreasing output efficiency away from a central
maximum. FIG. 2 shows how the temporal pulse width (pulse length or
duration) of KrF laser pulses decrease with increasing F.sub.2
concentration. FIGS. 3-4 show the dependence of output energy on
driving voltage (i.e., of the discharge circuit) for various
F.sub.2 concentrations of a F.sub.2 laser. It is observed from
FIGS. 3-4 that for any given driving voltage, the pulse energy
decreases with decreasing F.sub.2 concentration. In FIG. 3, for
example, at 1.9 kV, the pulse energies are around 13 mJ, 11 mJ and
10 mJ for F.sub.2 partial pressures of 3.46 mbar, 3.16 mbar and
2.86 mbar, respectively. The legend in FIG. 3 indicates the partial
pressures of two premixes, i.e., premix A and premix B, that are
filled into the discharge chamber of a KrF laser. Premix A
comprised substantially 1% F.sub.2 and 99% Ne, and premix B
comprised substantially 1% Kr and 99% Ne. Therefore, for the graph
indicated by triangular data points, a partial pressure of 346 mbar
for premix A indicates that the gas mixture had substantially 3.46
mbar of F.sub.2 and a partial pressure of 3200 mbar for premix B
indicates that the gas mixture had substantially 32 mbar of Kr, the
remainder of the gas mixture being the buffer gas Ne. FIG. 5 shows
a steadily increasing bandwidth of a KrF laser with increasing
F.sub.2 concentration.
[0006] In industrial applications, it is advantageous to have an
excimer or molecular fluorine laser capable of operating
continuously for long periods of time, i.e., having minimal
downtime. It is desired to have an excimer or molecular laser
capable of running non-stop year round, or at least having a
minimal number and duration of down time periods for scheduled
maintenance, while maintaining constant output beam parameters.
Uptimes of, e.g., greater than 98% require precise control and
stabilization of output beam parameters, which in turn require
precise control of the composition of the gas mixture.
[0007] Unfortunately, gas contamination occurs during operation of
excimer and molecular fluorine lasers due to the aggressive nature
of the fluorine or chlorine in the gas mixture. The halogen gas is
highly reactive and its concentration in the gas mixture decreases
as it reacts, leaving traces of contaminants. The halogen gas
reacts with materials of the discharge chamber or tube as well as
with other gases in the mixture. Moreover, the reactions take place
and the gas mixture degrades whether the laser is operating
(discharging) or not. The passive gas lifetime is about one week
for a typical KrF-laser.
[0008] During operation of a KrF-excimer laser, such contaminants
as HF, CF.sub.4, COF.sub.2, SiF.sub.4 have been observed to
increase in concentration rapidly (see G. M. Jurisch et al., Gas
Contaminant Effects in Discharge-Excited KrF Lasers, Applied
Optics, Vol. 31, No. 12, pp. 1975-1981 (Apr. 20, 1992)). For a
static KrF laser gas mixture, i.e., with no discharge running,
increases in the concentrations of HF, O.sub.2, CO.sub.2 and
SiF.sub.4 have been observed (see Jurisch et al., above).
[0009] One way to effectively reduce this gas degradation is by
reducing or eliminating contamination sources within the laser
discharge chamber. With this in mind, an all metal, ceramic laser
tube has been disclosed (see D. Basting et al., Laserrohr fur
halogenhaltige Gasentladungslaser" G 295 20 280.1, Jan. 25,
1995/Apr. 18, 1996 (disclosing the Lambda Physik Novatube, and
hereby incorporated by reference into the present application)).
FIG. 6 qualitatively illustrates how using a tube comprising
materials that are more resistant to halogen erosion (plot B) can
slow the reduction of F.sub.2 concentration in the gas mixture
compared to using a tube which is not resistant to halogen erosion
(plot A). The F.sub.2 concentration is shown in plot A to decrease
to about 60% of its initial value after about 70 million pulses,
whereas the F.sub.2 concentration is shown in plot B to decrease
only to about 80% of its initial value after the same number of
pulses. Gas purification systems, such as cryogenic gas filters
(see U.S. Pat. Nos. 4,534,034, 5,136,605, 5,430,752, 5,111,473 and
5,001,721 assigned to the same assignee, and hereby incorporated by
reference) or electrostatic particle filters (see U.S. Pat. No.
4,534,034, assigned to the same assignee and U.S. Pat. No.
5,586,134, each of which is incorporated by reference) are also
being used to extend KrF laser gas lifetimes to 100 million shots
before a new fill is advisable.
[0010] It is not easy to directly measure the halogen concentration
within the laser tube for making rapid online adjustments (see U.S.
Pat. No. 5,149,659 (disclosing monitoring chemical reactions in the
gas mixture)). Therefore, it is recognized in the present invention
that an advantageous method applicable to industrial laser systems
includes using a known relationship between F.sub.2 concentration
and a laser parameter, such as one of the F.sub.2 concentration
dependent output beam parameters mentioned above. In such a method,
precise values of the parameter would be directly measured, and the
F.sub.2 concentration would be calculated from those values. In
this way, the F.sub.2 concentration may be indirectly
monitored.
[0011] Methods have been disclosed for indirectly monitoring
halogen depletion in a narrow band excimer laser by monitoring beam
profile (see U.S. Pat. No. 5,642,374, hereby incorporated by
reference) and spectral (band) width (see U.S. Pat. No. 5,450,436,
hereby incorporated by reference). Neither of these methods is
particularly reliable, however, since beam profile and bandwidth
are each influenced by various other operation conditions such as
repetition rate, tuning accuracy, thermal conditions and aging of
the laser tube. That is, the same bandwidth can be generated by
different gas compositions depending on these other operating
conditions.
[0012] An advantageous technique monitors amplified spontaneous
emission (ASE), as is described in U.S. Pat. No. 6,243,406
(assigned to the same assignee and hereby incorporated by
reference). The ASE is very sensitive to changes in fluorine
concentration, and thus the fluorine concentration may be monitored
indirectly by monitoring the ASE, notwithstanding whether other
parameters are changing and affecting each other as the fluorine
concentration in the gas mixture changes.
[0013] It is known to compensate the degradation in laser
efficiency due to halogen depletion by steadily increasing the
driving voltage of the discharge circuit to maintain the output
beam at constant energy. To illustrate this, FIG. 7 shows how, at
constant driving voltage, the energy of output laser pulses
decreases with pulse count. FIG. 8 then shows how the driving
voltage may be steadily increased to compensate the halogen
depletion and thereby produce output pulses of constant energy.
[0014] One drawback of this approach is that output beam parameters
other than energy such as those discussed above with respect to
FIGS. 1-5 affected by the gas mixture degradation will not be
correspondingly corrected by steadily increasing the driving
voltage. FIGS. 9-11 illustrate this point showing the driving
voltage dependencies, respectively, of the long and short axis beam
profiles, short axis beam divergence and energy stability sigma.
Moreover, at some point the halogen becomes so depleted that the
driving voltage reaches its maximum value and the pulse energy
cannot be maintained without refreshing the gas mixture.
[0015] It is desired to have a method of stabilizing all of the
output parameters affected by halogen depletion and not just the
energy of output pulses. It is recognized in the present invention
that this is most advantageously achieved by adjusting the halogen
and rare gas concentrations themselves.
[0016] There are techniques available for replenishing a gas
mixture by injecting additional rare and halogen gases into the
discharge chamber between new gas fills and to methods including
readjusting the gas pressure, e.g., by releasing gases from the
laser tube (see especially U.S. Pat. Nos. 6,490,307 and 6,212,214,
and also U.S. Pat. No. 6,243,406; and U.S. Pat. Nos. 5,396,514 and
4,977,573, each of which is assigned to the same assignee and
hereby incorporated by reference). A more complex system monitors
gas mixture degradation and readjusts the gas mixture using
selective replenishment algorithms for each gas of the gas mixture
(see U.S. Pat. No. 5,440,578, hereby incorporated by reference).
One technique uses an expert system including a database of
information and graphs corresponding to different gas mixtures and
laser operating conditions (see the '214 patent, mentioned just
above). A data set of driving voltage versus output pulse energy,
e.g., is measured and compared to a stored "master" data set
corresponding to an optimal gas composition such as may be present
in the discharge chamber after a new fill. From a comparison of
values of the data sets and/or the slopes of graphs generated from
the data sets, a present gas mixture status and appropriate gas
replenishment procedures, if any, may be determined and undertaken
to re-optimize the gas mixture. Early gas replenishment procedures
are described in the '573 patent (mentioned above).
[0017] Most conventional techniques generally produce some
disturbances in laser operation conditions when the gas is
replenished. For example, strong pronounced jumps of the driving
voltage are produced as a result of macro-halogen injections
(macro-HI) as illustrated in FIG. 12 (macro-HI are distinguished
from micro-halogen injections, or .mu.HI, as described in the '307
patent). The result of a macro-HI is a strong distortion of
meaningful output beam parameters such as the pulse-to-pulse
stability. For this reason, in some techniques, the laser is
typically shut down and restarted for gas replenishment, remarkably
reducing laser uptime (see U.S. Pat. No. 5,450,436).
[0018] The '307 patent referred to above provides a technique
wherein gas replenishment is performed for maintaining constant gas
mixture conditions without disturbing significant output beam
parameters. The '307 patent describes a gas discharge laser system
which has a discharge chamber containing a gas mixture including a
constituent halogen-containing species, a pair of electrodes
connected to a power supply circuit including a driving voltage for
energizing the first gas mixture, and a resonator surrounding the
discharge chamber for generating a laser beam.
[0019] A gas supply unit is connected to the discharge chamber for
replenishing the gas mixture including the constituent
halogen-containing species. The gas supply unit includes a gas
inlet port having a valve for permitting a small amount of gas to
inject into the discharge chamber to mix with the gas mixture
therein. A processor monitors a parameter indicative of the partial
pressure of the first constituent gas and controls the valve at
successive predetermined intervals to compensate a degradation of
the constituent halogen-containing species in the gas mixture.
[0020] The partial pressure of the halogen containing-species in
the gas mixture is increased by an amount preferably less than 0.2
mbar, as a result of each successive injection. The gaseous
composition of the injected gas is preferably 1%-5% of the
halogen-containing gas and 95%-99% buffer gas, so that the overall
pressure in the discharge chamber increases by less than 20 mbar,
and preferably less than 10 mbar per gas injection.
[0021] The processor monitors the parameter indicative of the
partial pressure of the halogen-containing gas and the parameter
varies with a known correspondence to the partial pressure of the
halogen gas. The small gas injections each produce only small
variations in partial pressure of the halogen gas in the gas
mixture of the laser tube, and thus discontinuities in laser output
beam parameters are reduced or altogether avoided.
[0022] The constituent gas is typically a halogen containing
molecular species such as molecular fluorine or hydrogen chloride.
The constituent gas to be replenished using the method of the '307
patent may alternatively be an active rare gas or gas additive. The
monitored parameter may be any of time, shot count, driving voltage
for maintaining a constant laser beam output energy, pulse shape,
pulse duration, pulse stability, beam profile, bandwidth of the
laser beam, energy stability, temporal pulse width, temporal
coherence, spatial coherence, amplified spontaneous emission (ASE),
discharge width, and long and short axial beam profiles and
divergences, or a combination thereof. Each of these parameters
varies with a known correspondence to the partial pressure of the
halogen, and then halogen partial pressure is then precisely
controlled using the small gas injections to provide stable output
beam parameters.
[0023] The gas supply unit of the '307 patent preferably includes a
small gas reservoir for storing the constituent gas or second gas
mixture prior to being injected into the discharge chamber (see
U.S. Pat. No. 5,396,514, which is assigned to the same assignee and
is hereby incorporated by reference, for a general description of
how such a gas reservoir may be used). The reservoir may be the
volume of the valve assembly or an additional accumulator. The
accumulator is advantageous for controlling the amount of the gas
to be injected. The pressure and volume of the gases to be injected
are selected so that the overall pressure in the discharge chamber
will increase by a predetermined amount preferably less than 10
mbar, and preferably between 0.1 and 2 mbar, with each injection.
As above, the halogen partial pressure preferably increases by less
than 0.2 mbar and preferably far less such as around 0.02 mbar per
injection. These preferred partial pressures may be varied
depending on the percentage concentration of the halogen containing
species in the gas pre-mixture to be injected.
[0024] Injections may be continuously performed during operation of
the laser in selected amounts and at selected small intervals.
Alternatively, a series of injections may be performed at small
intervals followed by periods wherein no injections are performed.
The series of injections followed by the latent period would then
be repeated at predetermined larger intervals. A comprehensive
algorithm is desired for performing gas actions in order to better
stabilize the gas composition in the laser tube, and
correspondingly better stabilize significant parameters of the
output beam of the excimer or molecular fluorine laser system.
BRIEF SUMMARY
[0025] An excimer or molecular fluorine gas discharge laser system
is provided including a laser chamber containing a laser gas
mixture at least molecular fluorine and a buffer gas, the molecular
fluorine being particularly subject to depletion; a power supply
circuit including a high voltage power supply and a pulse
compression circuit; multiple electrodes connected to the power
supply circuit for providing a driving voltage as a pulsed
discharge to energize said laser gas mixture, the multiple
electrodes including a pair of main electrodes and at least one
preionization unit; a resonator including the discharge chamber and
line-narrowing and/or line-selection optics for generating a
pulsed, narrowband laser beam at a wavelength less than 250 nm and
a bandwidth less than 1 pm; a fan for circulating the gas mixture
between the main electrodes at a predetermined flow rate, wherein
the discharge width divided by the flow rate of said gas mixture
through said discharge is less than substantially 0.5 milliseconds;
a heat exchanger for controlling a temperature of the gas mixture;
a gas supply unit connected to the laser chamber; a processor for
controlling gaseous flow between said gas supply unit and the laser
chamber, wherein the gas supply unit and the processor are
configured to permit a quantity less than 7% of the halogen gas in
the laser chamber to inject into the laser chamber at selected
intervals; and an amplifier, wherein the narrowband laser beam
generated by the resonator is directed through the amplifier for
increasing the power of the beam.
[0026] Extra-resonator optics may be provided for redirecting the
beam generated by and outcoupled from the resonator back into the
laser chamber at or near a time of maximum discharge current within
the laser chamber, as the amplifier for increasing the power of the
beam. The extra-resonator optics may include an optical delay line
for timing the entry of the beam back into the laser chamber for
amplification at or near the time of maximum discharge current.
[0027] The buffer gas may include neon for pressurizing the gas
mixture sufficiently to enhance the performance of the laser, and
wherein the processor cooperates with the gas supply system to
control the molecular fluorine concentration within the discharge
chamber to maintain the molecular fluorine concentration within a
predetermined range of optimum performance of the laser.
[0028] An aperture may be provided within the resonator. The
line-narrowing and/or selection optics may include a beam expander
before at least one of a grating, a grism, an interferometric
device and a dispersion prism. The aperture may be positioned
between the laser chamber and the beam expander. A second aperture
may be provided on the other side of the laser chamber. A highly
reflective mirror may be provided before the grating.
[0029] An excimer or molecular fluorine gas discharge laser system
is also provided including a laser chamber containing a laser gas
mixture at least molecular fluorine and a buffer gas, the molecular
fluorine being particularly subject to depletion; a power supply
circuit including a high voltage power supply and a pulse
compression circuit; multiple electrodes connected to the power
supply circuit for providing a driving voltage as a pulsed
discharge to energize said laser gas mixture, the multiple
electrodes including a pair of main electrodes and at least one
preionization unit; a resonator including the discharge chamber and
line-narrowing and/or line-selection optics for generating a
pulsed, narrowband laser beam at a wavelength less than 250 nm and
a bandwidth less than 1 pm; a fan for circulating the gas mixture
between the main electrodes at a predetermined flow rate, wherein
the discharge width divided by the flow rate of said gas mixture
through said discharge is less than substantially 0.5 milliseconds;
a heat exchanger for controlling a temperature of the gas mixture;
a gas supply unit connected to the laser chamber; a processor for
controlling gaseous flow between the gas supply unit and the laser
chamber, wherein the gas supply unit and the processor are
configured to permit a quantity less than 7% of the halogen gas in
the laser chamber to inject into the laser chamber at selected
intervals; and an energy detector module including an energy
detector and beam splitter module provided in a sealed enclosure
substantially devoid of molecular species that photoabsorb around
the sub-250 nm wavelength of the narrowband laser beam, and wherein
the beam splitter module separates a beam portion from a main
output laser beam for detection at the energy detector.
[0030] The energy detector module may be purged with an inert gas
at a slight, regulated overpressure, or evacuated to low pressure.
The energy detector module may be coupled with a main enclosure for
the narrowband laser beam, such that a beam path of the separated
beam portion to be detected at the energy detector is substantially
free of the photoabsorbing species.
[0031] An excimer or molecular fluorine gas discharge laser system
is further provided including a laser chamber containing a laser
gas mixture at least molecular fluorine and a buffer gas, the
molecular fluorine being particularly subject to depletion; a power
supply circuit including a high voltage power supply and a pulse
compression circuit; multiple electrodes connected to the power
supply circuit for providing a driving voltage as a pulsed
discharge to energize the laser gas mixture, the multiple
electrodes including a pair of main electrodes and at least one
preionization unit, wherein at least one of the main electrodes
includes a narrow central portion and a base portion, the narrow
portion substantially carrying a discharge current such that the
discharge width is substantially 4 mm or less; a resonator
including the discharge chamber and line-narrowing and/or
line-selection optics for generating a pulsed, narrowband laser
beam at a wavelength less than 250 nm and a bandwidth less than 1
pm; a fan for circulating the gas mixture between the main
electrodes at a predetermined flow rate, wherein the discharge
width divided by the flow rate of the gas mixture through the
discharge is less than substantially 0.5 milliseconds; a heat
exchanger for controlling a temperature of the gas mixture; a gas
supply unit connected to the laser chamber; and a processor for
controlling gaseous flow between the gas supply unit and the laser
chamber, wherein the gas supply unit and the processor are
configured to permit a quantity less than 7% of the halogen gas in
the laser chamber to inject into the laser chamber at selected
intervals.
[0032] The laser gas flow rate may be more than 10-15 m/s. The
discharge width may be 2 mm or less. The laser chamber may include
a spoiler for forming gas flow between the main electrodes to
reduce turbulence. The laser chamber may further include
aerodynamic current return ribs defining upstream to downstream
tapered openings for further forming gas flow between the main
electrodes to further reduce turbulence. The discharge width
divided by the flow rate may be less than or equal to substantially
0.25 milliseconds.
[0033] A method is provided for controlling a composition of a gas
mixture within a laser chamber of a high power (2 kHz or more)
excimer or molecular fluorine gas discharge laser system including
the laser chamber disposed within a laser resonator including
line-narrowing and/or selection optics, and an amplifier chamber,
the gas mixture at least including molecular fluorine and a buffer
gas. The method includes operating the laser system for generating
a high power, narrowband laser beam; monitoring a parameter
indicative of the molecular fluorine concentration in the gas
mixture; determining a next amount of molecular fluorine less than
substantially 7% of an amount already in the laser chamber to be
injected into the laser chamber based on an amount determined at
least approximately to be within the laser chamber; narrowing the
bandwidth of the beam to less than 1 pm within the laser resonator;
outcoupling the beam from the resonator; and amplifying the
outcoupled beam within the amplifier chamber for increasing the
power of the beam.
[0034] A step of monitoring an input driving voltage of a pulse
power circuit of the laser system may be including, as well as
determining the next amount of molecular fluorine based further on
a value of the input driving voltage. A step of adjusting a total
pressure of the gas mixture within the laser tube to maintain the
input driving voltage within a tolerance range of an optimal input
driving voltage may also be included. The total pressure adjusting
step may include releasing a predetermined amount of the gas
mixture from the laser tube and/or adding a predetermined amount of
gas to the gas mixture within the laser tube. The method may
further include applying a first input voltage to the electrodes to
excite the gas mixture having a first pressure for generating the
beam at the desired energy; and applying a second input voltage to
the electrodes to excite the gas mixture having a second pressure
for generating the beam at the substantially same desired energy.
The narrowing step may include expanding and dispersing the beam
prior to the outcoupling step. The narrowing step may further
include passing the beam through one or more intra-resonator
apertures. The next amount to be injected may be less than
substantially 5% of said amount already in the laser chamber.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] FIG. 1 is a graph of the output efficiency of an excimer or
molecular laser versus F.sub.2-concentration.
[0036] FIG. 2 is a graph of integrated pulse width of an excimer or
molecular laser versus F.sub.2-concentration.
[0037] FIG. 3 shows several graphs of output beam energy of a KrF
excimer laser versus driving voltage for various gas mixture
component partial pressures.
[0038] FIG. 4 shows several graphs of output beam energy of an
excimer or molecular fluorine laser versus driving voltage for
various F.sub.2 concentrations.
[0039] FIG. 5 is a graph of the bandwidth of an excimer laser
versus F.sub.2 concentration.
[0040] FIG. 6 illustrates how F.sub.2 depletion rates vary for
excimer or molecular fluorine lasers depending on discharge chamber
composition.
[0041] FIG. 7 is a graph of pulse energy versus pulse count for an
excimer or molecular laser operating at constant driving
voltage.
[0042] FIG. 8 is a graph of driving voltage versus pulse count for
an excimer or molecular laser operating at constant output pulse
energy.
[0043] FIG. 9 shows a first graph of the long axis beam profile
versus driving voltage and a second graph of the short axis beam
profile versus driving voltage for an excimer or molecular laser
operating at constant output pulse energy.
[0044] FIG. 10 is a graph of the divergence of the short axis of an
output beam versus driving voltage of an excimer or molecular laser
operating at constant output pulse energy.
[0045] FIG. 11 is a graph of output pulse energy stability versus
driving voltage of an excimer or molecular laser operating at
constant output pulse energy.
[0046] FIG. 12 illustrates the strong pronounced discontinuities in
the driving voltage when large halogen partial pressures increases
are rapidly effected in the discharge chamber due to halogen
injections.
[0047] FIG. 13a shows a schematic block diagram of an excimer or
molecular laser in accord with a preferred embodiment.
[0048] FIG. 13b shows a schematic diagram of the gas control unit
of the excimer or molecular laser of FIG. 13a.
[0049] FIG. 14a schematically shows gas lines for halogen
injections into the discharge chamber of the laser of FIG. 13 using
an accumulator.
[0050] FIG. 14b shows a computer display connected to the processor
of FIG. 13a indicating that the processor is controlling the gas
replenishment process.
[0051] FIG. 15 is a graph of driving voltage versus time also
showing periodic halogen injections for a system in accord with a
preferred embodiment.
[0052] FIG. 16 is a graph of driving voltage versus time also
showing periodic halogen injections and mini gas replacements for a
system in accord with a preferred embodiment.
[0053] FIG. 17 is a graph of pulse energy stability (sigma, upper
graph) versus time and moving averages (over 40 pulse intervals,
maximum and minimum) for a laser system operating at 2 kHz in
accord with a preferred embodiment.
[0054] FIG. 18 is qualitative graph of driving voltage versus time
also showing periodic micro-halogen injections (.mu.HI) for a
system in accord with a preferred embodiment.
[0055] FIG. 19 is a graph of energy stability variation versus
pulse count for a system in accord with a preferred embodiment.
[0056] FIG. 20 is a graph of beam divergence versus pulse count for
a system in accord with a preferred embodiment.
[0057] FIG. 21 is a qualitative graph of driving voltage versus
pulse count also showing periodic halogen injections, mini gas
replacements and partial gas replacements for a system in accord
with a preferred embodiment.
[0058] FIG. 22 is a flow diagram for performing halogen injections,
mini gas replacements and partial gas replacements in accord with a
preferred embodiment.
[0059] FIG. 23 is a further qualitative graph of driving voltage
versus pulse count also showing periodic halogen injections, mini
gas replacements and partial gas replacements for a system in
accord with a preferred embodiment.
[0060] FIG. 24 is a further flow diagram for performing halogen
injections, mini gas replacements and partial gas replacements in
accord with a preferred embodiment.
[0061] FIG. 25 schematically shows an excimer or molecular fluorine
laser system according to a preferred embodiment.
[0062] FIG. 26 shows a gas action algorithm including a pressure
release step according to a first preferred embodiment.
[0063] FIG. 27 shows a gas action algorithm including a pressure
release step according to a second preferred embodiment.
[0064] FIG. 28 shows plots characterizing laser performances for
different values of the quality factor Q.
[0065] FIG. 29 shows a gas action algorithm including pressure
release and pressure addition steps according to a third
embodiment.
[0066] FIGS. 30a-30f schematically show several alternative
embodiments in accord with a first aspect of the invention
including various line narrowing resonators and techniques
utilizing line-narrowed oscillators for the molecular fluorine
laser.
[0067] FIG. 31a schematically shows a preferred embodiment in
accord with a second aspect of the invention including an
oscillator, a spectral filter in various configurations, and an
amplifier.
[0068] FIGS. 31b-31d schematically show alternative embodiments of
spectral filters in further accord with the second aspect of the
invention.
[0069] FIG. 32a schematically shows an alternative embodiment in
accord with the second aspect of the invention including a single
discharge chamber providing the gain medium for both an oscillator
and an amplifier, and having a spectral filter in between.
[0070] FIG. 32b(i)-(iii) respectively show waveforms of the
electrical discharge current, un-narrowed beam intensity and output
beam intensity in accord with the alternative embodiment of FIG.
3a.
[0071] FIG. 33a schematically shows a preferred embodiment in
accord with a third aspect of the invention including a
line-narrowed oscillator followed by a power amplifier.
[0072] FIGS. 33b-33f schematically show alternative embodiments of
line-narrowed oscillators in further accord with the third aspect
of the invention.
[0073] FIGS. 34a-34b schematically show alternative embodiments in
accord with a fourth aspect of the invention including a single
discharge chamber providing the gain medium for both an oscillator
with line-narrowing and an amplifier.
[0074] FIG. 35 shows an energy detector for use with a F.sub.2
laser system in accord with the fifth aspect of the invention.
[0075] FIG. 36a shows a discharge chamber for a F.sub.2 laser in
accord with a seventh aspect of the invention.
[0076] FIG. 36b shows a cross sectional view of the ribs crossing
the gas flow of the laser tube where the gas flows into the
discharge chamber from the gas flow vessel, wherein the ribs are
separated by openings to permit the gas flow and aerodynamically
shaped to provide more uniform gas flow and the ribs further serve
as low inductivity current return bars.
[0077] FIG. 36c shows a cross sectional view of the ribs crossing
the gas flow of the laser tube separated by openings to permit gas
flow from the discharge chamber back into the gas flow vessel,
wherein the ribs are aerodynamically shaped and separated by
openings through which gas exits the discharge chamber and flows
back into the gas flow vessel.
INCORPORATION BY REFERENCE
[0078] What follows is a cite list of references each of which is,
in addition to those references cited above in the priority
section, hereby incorporated by reference into the detailed
description of the preferred embodiment below, as disclosing
alternative embodiments of elements or features of the preferred
embodiments not otherwise set forth in detail below. A single one
or a combination of two or more of these references may be
consulted to obtain a variation of the preferred embodiments
described in the detailed description below. Further patent, patent
application and non-patent references are cited in the written
description and are also incorporated by reference into the
preferred embodiment with the same effect as just described with
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62-160783; [0091] R. Hultzsch: Gitterprismen, Photonik (September
1998), p. 40; [0092] W. Demtroder: Laser Spectroscopy Springer,
Berlin Heidelberg (1996) p. 112; and [0093] W. A. Taub: Constant
Dispersion Grism Spectrometer for Channeled Spectra J. Opt. Soc.
Am. A7 (1990) p. 1779.
DETAILED DESCRIPTION
[0094] A gas replenishment technique is provided for an excimer or
molecular fluorine laser system. The technique encompasses several
aspects of the present invention, each contributing to achieving
the above objects. In a first aspect, it is recognized that the
fluorine concentration in the laser gas mixture has a known
correspondence to the value of the driving voltage, when the
driving voltage is being adjusted to maintain a constant
pulse-to-pulse output beam energy, constant energy dose or moving
average energy dose, optimum energy stability, etc. Thus, a
particular gas replenishment action is performed first based on the
value of the driving voltage for each gas action, and then based on
a counter that counts total accumulated electrical input to the
discharge, time and/or pulse count.
[0095] For example, the amount of gas including a
halogen-containing species and/or the total amount of gas injected
may be based on the driving voltage. Whether the gas action is a
partial or mini gas replacement or only a gas injection is also
determined based on the driving voltage. It may be determined that
no gas action will be presently performed. Also, the interval
between the previous gas action and the next gas action may be
adjusted.
[0096] Another factor that is preferably taken in account in
determining the above particulars of the next gas action is the
specific amount of halogen that was injected during the previous
gas action. That amount may be determined based on measurements of
the gas pressure in an accumulator (see the '785 application) from
which the gas was injected during the previous gas action (and
optionally also based on the pressure in the laser tube). The
temperatures of the gas mixtures in the laser tube and the
accumulator may also be taken into account.
[0097] On a larger overall scale, or macro scale, the determination
of which gas actions are to be performed, if any, may be based on
which of several ranges of driving voltages that the driving
voltage is presently at. For example, if the driving voltage is
presently in a first range, then partial gas replacement (PGR) will
be performed for cleaning the gas mixture, and causing the driving
voltage to vary out of the first range.
[0098] If the driving voltage is presently in a second range below
the first range, then enhanced .mu.HIs together with periodic
mini-gas replacements (MGR) are performed, preferably subject to
adjustments as described above from injection to injection and/or
from MGR to MGR, until the driving voltage varies out of the second
range. Enhanced .mu.HIs may include injections of larger amounts of
halogen than ordinary .mu.HIs, or the injections may be performed
more often or at reduced intervals than ordinary .mu.HIs would be
performed.
[0099] If the driving voltage is presently in a third range below
the second range, then ordinary .mu.HIs together with periodic
mini-gas replacements (MGR) are performed, preferably subject to
adjustments as described above from injection to injection and/or
from MGR to MGR, until and unless the driving voltage varies out of
the third range.
[0100] If the driving voltage is presently in a fourth range below
the third range, then no gas actions are performed. Alternatively,
a gas replacement action may be performed, e.g., to reduce the
fluorine concentration in the gas mixture. More than MGR may be
performed, or more than one amount of gas may be injected (and
correspondingly released) during MGRs, as well, or the interval
between MGRs may be adjusted.
[0101] In addition, after a new fill of the laser tube, the system
of the present invention is adjusted depending on the age of the
tube and/or the optics of the laser resonator. The driving voltage
ranges may be adjusted within which the particular types of gas
actions are performed as described above.
[0102] FIG. 13a shows a schematic block diagram of a preferred
embodiment of an excimer or molecular fluorine laser. The laser
system of FIG. 13a includes a laser tube 1 including an electrode
or discharge chamber and a gas flow vessel, wherein the gas flow
vessel typically includes a blower and heat exchanger or cooling
unit. The laser tube 1 contains a laser gas mixture, and a pressure
gauge P is preferably provided for monitoring the pressure in the
laser tube 1. A resonator surrounds the tube 1 and includes a rear
optics module 2 and a front optics module 3.
[0103] The rear optics module 2 includes a resonator reflector
which may be a highly reflective mirror, a grating or a highly
reflecting surface of another optical component such as an etalon
or a prism. A wavelength calibration module is preferably included
with the rear optics module. Preferred wavelength calibration units
or devices and techniques are disclosed in U.S. Pat. No. 4,905,243
and U.S. patent application Ser. Nos. 09/136,275, 09/167,657 and
09/179,262, each of which is assigned to the same assignee as the
present application and is hereby incorporated by reference.
[0104] The front optics module 3 preferably includes a resonator
reflector which is preferably an output coupler. The resonator
reflector of the front optics module may alternatively be a highly
reflecting mirror and other means for output coupling the beam 13
may be used, such as a beam splitter or other angled partially
reflecting surface within the resonator. The front optics module 3
also may include a line narrowing and/or selection unit and/or a
wavelength tuning unit.
[0105] Alternatively, the line narrowing and/or selection unit
and/or wavelength tuning unit may be included with the rear optics
module. Such optical elements as one or more beam expanding
elements such as beam expanding prism(s) and/or lens arrangements,
one or more dispersive elements such as dispersive prism(s) and/or
a grating, one or more etalons, birefringent plate(s), or grism(s)
may be included for line narrowing, selection and/or tuning. U.S.
Pat. Nos. 4,399,540, 4,905,243, 5,226,050, 5,559,816, 5,659,419,
5,663,973, 5,761,236, and 5,946,337, and U.S. patent application
Ser. Nos. 09/317,695, 09/130,277, 09/244,554, 09/317,527,
09/073,070, 60/124,241, 60/140,532, and 60/140,531, each of which
is assigned to the same assignee as the present application, and
U.S. Pat. Nos. 5,095,492, 5,684,822, 5,835,520, 5,852,627,
5,856,991, 5,898,725, 5,901,163, 5,917,849, 5,970,082, 5,404,366,
4,975,919, 5,142,543, 5,596,596, 5,802,094, 4,856,018, and
4,829,536, are each hereby incorporated by reference into the
present application, as describing line narrowing, selection and/or
tuning elements, devices and/or techniques, among others known to
those skilled in the art, which may be used in a laser system
according to the preferred embodiment.
[0106] Wavelength, pulse energy, and gas control information, as
well as other information about the laser system is received by a
processor 11. The processor 11 controls the wavelength of the
output beam 13 by controlling the line tuning module based on the
wavelength information the processor 11 receives, the electrical
pulse power and discharge module ("pulse power module") 5 based on
pulse energy information it receives, and gas control elements 6-10
and 12 based on information it receives relating to the gas mixture
status, and on data saved in its database(s) (see the '653
application, above).
[0107] A beam portion is preferably received by an energy monitor 4
which measures the energy and/or angular distribution and/or other
beam parameters of the received beam portion of the output beam 13.
Data corresponding to the energy of the beam portion is then sent
to the processor 11 which is connected to the energy monitor 4. The
processor 11 then uses this information to perform processing
relating to the energy of the output beam 13.
[0108] The pulse power module 5 provides energy to the gas mixture
via a pair of electrodes 14 within the discharge chamber 1.
Preferably, a preionization unit (not shown) is also energized by
the pulse power module for preionizing the gas mixture just prior
to the main discharge. The energy of the output beam 13 of the
laser system has a known dependence on the "driving voltage" of the
pulse power module. The driving voltage is adjusted during laser
operation to control and stabilize the energy of the output beam
13. The processor 11 controls the driving voltage based on the
energy information received from the energy monitor 4. In accord
with the present invention, the processor 11 also controls and
stabilizes the status of the gas mixture and thus indirectly
controls and stabilizes other laser output beam parameters such as
energy stability, temporal pulse width, spatial and temporal
coherences, bandwidth, and long and short axial beam profiles and
divergences by controlling the status of the gas mixture within the
laser tube 1.
[0109] FIG. 13b shows a detailed schematic of the gas control box
10 of FIG. 13a. The gas control box 10 is connected to the laser
tube 1 for supplying gas based on control signals received from the
processor 11. The processor 11 regulates the delivery of gases or
mixtures of gases to the laser tube 1 via a valve assembly 6 or
system of valves. The valve assembly preferably has a reservoir or
compartment 7 having a known volume and having a pressure gauge P
attached for measuring the pressure in the compartment 7. The
compartment and the laser tube preferably also each have means,
such as a thermocouple arrangement, for measuring the temperature
of the gases within the compartment and tube. The compartment 7 may
be 20 cm.sup.3 or so in volumetric size (by contrast, the laser
tube 1 may be 42,000 cm.sup.3 volumetrically). Four valves 8a-8d
are shown as controlling the flow of gases contained in external
gas containers into the compartment 7. Of course, more or less than
four such valves may be provided. Another valve 32 is shown
controlling the access of a vacuum pump vp to the compartment 7
which is shown connected through a halogen filter hf. Another valve
34 is shown controlling the flow of gases between the compartment 7
and the laser tube 1. A further valve or valves (not shown) may be
provided along the line 35 from valve 34 to the tube 1 for
controlling the atmosphere in the line 35, e.g., using a pump for
evacuating the line 35.
[0110] Small amounts of a gas or gas mixture are preferably
injected from the compartment 7 into the discharge chamber 1 as
.mu.HIs or enhanced .mu.HIs, or during a PGR or MGR action. As an
example, the gas supply connected to the valve assembly 6 through
gas line 36a may be a premix A including 1% F.sub.2:99% Ne, and
that through gas line 36b may be a premix B including 1% Kr:99% Ne,
for a KrF laser. For an ArF laser, premix B would have Ar instead
of Kr, and for a F.sub.2 laser premix B is not used. Thus, by
injecting premix A and premix B into the tube 1 via the valve
assembly, the fluorine and krypton concentrations in the laser tube
1, respectively, may be replenished. Gas lines 36c and 36d may be
used for different additional gas mixtures. Although not shown, the
tube 1 preferably has additional means for releasing gas, or
alternatively, the gas is released through the valve assembly, such
as via valves 34 and 32.
[0111] New fills, partial and mini gas replacements and gas
injection procedures, e.g., enhanced and ordinary micro-halogen
injections, and any and all other gas replenishment actions are
initiated and controlled by the processor 11 which controls the
valve assembly 6 and the pump vp based on various input information
in a feedback loop.
[0112] An exemplary method according to the present invention is
next described for accurately and precisely replenishing the
fluorine concentration in the laser tube 1 in small amounts such
that significant output beam parameters are not significantly
disturbed, if at all, with each gas injection. The processor 11,
which is monitoring a parameter indicative of the fluorine
concentration in the laser tube 11, determines that it is time for
a micro-halogen injection (.mu.HI).
[0113] The processor 11 then sends a signal that causes valve 8a to
open and allow premix A to fill the compartment 7 to a
predetermined pressure, e.g., 5 bar. Then, valve 8a is closed and
valve 34 is opened allowing at least some of the premix A that was
filled into the compartment 7 to release into the laser tube 1.
[0114] If the pressure in the tube was 3 bar prior to the injection
and the tube has 42,000 cm.sup.3, and the injection is such that
the pressure in the accumulator was reduced to 3 bar after the
injection, then 2.times.20/40,000 bar would be the pressure
increase in the tube 1 as a result of the injection, or 1 mbar. If
the premix A contains 1% F.sub.2:99% Ne, then the increase in
partial pressure of the F.sub.2 in the laser tube as a result of
the injection would be approximately 0.01 mbar.
[0115] The above calculation may be performed by the processor 11
to determine more precisely how much F.sub.2 was injected, or prior
to injection, the pressure in the compartment 7 may be set
according to a calculation by the processor 11 concerning how much
F.sub.2 should be injected based on the status information of the
monitored parameter received by the processor 11, or based on
pre-programmed criteria. A correction for difference in temperature
between the gas in the compartment 7 and that in the tube 1 may
also be performed by the processor 11 for more accuracy, or the
temperature of the gas in the compartment 7 may be preset, e.g., to
the temperature within the laser tube 1.
[0116] Preferably, an amount of gas premix corresponding to smaller
than 10 mbar total gas pressure, or 0.1 mbar F.sub.2 partial
pressure, increase in the tube 1 is injected from the compartment
7. Even more preferably, less than 5 mbar or even 2 mbar total gas
pressure (0.05 or 0.02 mbar F.sub.2 partial pressure) increase in
the laser tube 1 results from the gas injection.
[0117] The compartment 7 may simply be the valve assembly 6 itself,
or may be an additional accumulator (described in detail below).
The compartment 7 is also configured so that the small amounts of
gas may be injected at successive very short intervals, to
compensate a degradation of a halogen gas and/or another gas or
gases within the discharge chamber 1 of an excimer or molecular
laser such as a KrF, ArF or F.sub.2 laser.
[0118] There may be more than one compartment like compartment 7,
as described above, each having different properties such as
volumetric space. For example, there may be two compartments, one
for .mu.HIs and the other for enhanced .mu.HIs. There may be more
than two, for still further versatility in the amounts of halogen
to be injected in a gas action, and for adjusting the driving
voltage ranges corresponding to different gas action algorithms.
Different premixes may be injected from the different compartments.
Also, the exemplary method described using premixes of particular
gas compositions, but many different gas compositions could be used
in accord with the present invention. For example, gas compositions
having higher fluorine (or hydrogen chloride) percentage
concentrations could be used such as 5% or 2% instead of 1%. There
also may be an additional valve connected to a 100% buffer gas
container.
[0119] Advantageously, the processor 11 and gas supply unit are
configured to permit the delivery or injection of very small
amounts of one or more gases or gas mixtures to the discharge
chamber 1. The injection of the small amounts of the gas or gas
mixture result in gas pressure increases in the discharge chamber 1
below 10 mbar, and preferably between 0.1 and 2 mbar. Each gas in
the gas mixture within the discharge chamber 1 may be separately
regulated so the gas composition within the discharge chamber may
be precisely controlled. For example, similar injections of Kr, Ar
or Xe may be performed for replenishing those gases in the laser
tube 1.
[0120] Because the amount of gas injected during a gas injection or
replacement procedure is small, laser output beam parameters do not
vary greatly with each injection. The injections are preferably
carried out periodically at predetermined intervals corresponding
to known depletion amounts of the gases. For example, if the
halogen partial pressure in the gas mixture of an F.sub.2 laser is
known, under current operating conditions, to be around 3 bar after
a new fill and to deplete by 0.1 mbar per X minutes or Y shots,
then halogen injections including, e.g., 1 mbar (pressure increase
in tube 1) of a premix including 1% F.sub.2 could be performed
every X/10 minutes or Y/10 shots, in accord with the present
invention, to maintain the concentration of the halogen, or halogen
injections of 2 mbar of the premix may be performed every X/5
minutes, and so on. Also, micro-halogen injections (.mu.HI) of 1
mbar of premix A including 1% F.sub.2 and 99% Ne buffer may be
injected every X/5 minutes for 100 minutes followed by a period of
100 minutes when no injections are performed. Many variations are
possible within the spirit of the present invention including
irregular gas actions as determined by the processor.
[0121] In contrast with the present invention, if, e.g., a 50 mbar
(pressure increase in tube 1) premix A injection (again having 1%
F.sub.2 such that the F.sub.2 partial pressure increase in the tube
1 is 0.5 mbar and corresponds to around a 17% increase in the
F.sub.2 concentration in the tube 1) is performed every 5X minutes
or 5Y shots, or at any time, the large injection amount will cause
output beam parameters of the laser beam to noticeably and
undesirably fluctuate in response. For example, the pulse energy or
driving voltage can fluctuate by 10% or more when the large
injection is performed. If the laser is not shut down, or
industrial processing interrupted, when the large injection is
performed, then imprecise industrial processing will occur due to
disturbances in meaningful output beam parameters.
[0122] The halogen injection algorithm of the present invention may
be considered to extend a total halogen injection over a longer
period of time or number of pulse counts. Over the period of the
several halogen injections, the high voltage and the F.sub.2
concentration do not change significantly so that significant
changes in pulse energy and pulse energy stability, among other
meaningful output beam parameters, are eliminated. Again, some of
these other output beam parameters are listed above and each will
be extremely stable using the method of the present invention.
[0123] FIG. 14a schematically shows another configuration of gas
lines for halogen injections into the discharge chamber 1 of the
laser of FIG. 13a using an accumulator 6a. The accumulator 6a is
connected to the laser tube 1 via laser head valve LH. The
accumulator 6a is also connected to a gas line 12a via halogen
valve H connected to a gas bottle 13 including the halogen or
halogen premix. For example, the gas bottle 13 may be filled with a
gas mixture including an F.sub.2 mixture (e.g., 5% F.sub.2/95% Ne
or a 5% HCl/1% H.sub.2 in neon mixture or a 1% F.sub.2:99% Ne
premix, among other possibilities). A pump is shown connected to
each of the accumulator 6a and the laser tube 1 via a vacuum valve
V. The tube 1 is shown valve-connected to additional gas lines and
valves including a buffer gas via valve B, a rare gas or rare gas
premix via valve R (used with KrF, ArF, XeCl and XeF excimer
lasers, e.g.) and an inert gas via valve I. The inert gas valve I
or another valve not depicted may be used for valve connecting to a
source of Xe to be used as an additive in the gas mixture within
the tube. Again, one or more additional accumulators may be added
to the system.
[0124] The accumulator 6a has the particular advantage that the
small amounts of gas including the F.sub.2 within the F.sub.2
premix to be injected with each halogen injection in accord with
the present invention may be precisely controlled. The accumulator
is easily pumped to low pressure. A precise amount of F.sub.2 gas
or F.sub.2 gas premix is released into the accumulator 6a and the
amount of F.sub.2 is determined according to the total gas pressure
within the accumulator, the known volumes of the accumulator 6a and
the laser tube 1 and the known concentration of the F.sub.2 or the
F.sub.2 percentage concentration in the premix gas. A F.sub.2
partial pressure increase in the laser tube 1 after the injection
is determined based on the amount of F.sub.2 known to be in the
accumulator 6a prior to (and possible after) the injection.
[0125] Based on this determination and/or other factors such as the
interval between the previous and current gas actions (measured in
time or pulse count, e.g.) and/or the value of the driving voltage
at the time of the previous, present and/or next gas action, the
interval between the current and next gas action and/or the amount
of halogen containing gas or total gas to be injected in the next
gas action may be determined so that a precise amount of each gas,
particularly the halogen-containing gas, may be injected in the
next gas action. Also, the type of gas action to be performed may
be determined based on these or other factors.
[0126] FIG. 14b shows how a display monitor attached to the
processor 11 might look as the laser system is operating. The laser
tube is shown to have an internal pressure of 2064 mbar, while the
pressure within the gas manifold (corresponding to the compartment
7 of FIG. 13a or the accumulator 6a of FIG. 14a) shows an internal
pressure of 4706 mbar. As discussed, the precise amount of gas
injected into the laser tube can be calculated based in part on
these pressure readings. Again, the temperature may be taken into
account for making an even more precise determination.
[0127] Various gas actions and procedures will now be described.
The procedures are potentially applicable to all gas discharge
lasers, although excimer lasers (e.g., KrF, ArF, XeCl and XeF) and
the F.sub.2 laser would benefit greatly by the present invention.
The KrF-laser is used as a particular example below.
[0128] The process begins with a new fill which is performed prior
to operating the laser system. For a new fill, the laser tube 1 is
evacuated and a fresh gas mixture is then filled in. A new fill of
a KrF-laser would typically result in a gas mixture having
approximately the following partition of gases: F.sub.2: Kr:
Ne=0.1%: 1.0%: 98.9%. If the gas mixture within the KrF laser
discharge chamber has a typical total pressure of around p=3000
mbar, then the partial pressures of F.sub.2 and Kr would typically
be around 3 mbar and 30 mbar, respectively. A new fill for a
F.sub.2 laser would produce the following typical partition of
gases:F.sub.2:Ne=0.1% :99.9%. For the F.sub.2 laser, He or a
mixture of He and Ne may be used as the buffer instead of only Ne
(see the '526 application, above).
[0129] The new fill procedure can be performed using separate gas
lines delivering pure or premixed gases. Typical gas premixes used
regularly in semiconductor industry fabs are premixes A and B,
where: premix A has 1% F.sub.2/1% Kr/Ne and premix B has 1%
Kr/Ne.
[0130] After the new fill is performed, the halogen gas begins to
react with components of the laser tube 1 that it comes into
contact with, whether the laser is operating or not. "Gas
replenishment" is a general term which includes gas replacement
(PGRs and MGRs each subject to varying amounts and compositions of
injected and released gases) and gas injections (.mu.HIs and
enhanced .mu.HIs again each subject to varying amounts and
compositions of injected gases), performed to bring the gas mixture
status back closer to new fill status.
[0131] Any gas replenishment procedures are performed taking into
account that each gas in the gas mixture depletes at a different
depletion rate due to the halogen depletion just described and the
gas replenishment procedures performed in response. For the narrow
band KrF-laser, e.g., F.sub.2-depletion occurs at a rate of between
about 0.1% to 0.3% (and sometimes up to nearly 1%) per million
shots, whereas Kr depletion occurs about 10 to 50 times more
slowly. The Ne buffer is less important, but may also be considered
as part of an overall gas replenishment operation, e.g., to
maintain a desired pressure in the tube 1.
[0132] Separate gas actions are preferably performed to replenish
each constituent gas of the gas mixture. For the KrF-laser, for
example, the F.sub.2 may be replenished by halogen or halogen/rare
gas or premix A injections and the Kr replenished by rare gas or
premix B injections. Other gas additives such as Xe may be
replenished by Xe gas or still further premixes C, D, etc. The
individual depletion rates also depend on operating conditions of
the laser such as whether the laser is in broadband or narrow band
mode, the operating energy level, whether the laser is turned off
or is in continuous, standby or other burst pattern operation, and
the operating repetition rate. The processor 11 is programmed to
consider all of these variations in laser operation.
[0133] The gas mixture status is considered sufficiently stable in
the present invention when deviations in fluorine and krypton
content are below 5%, and preferably below 3%. Without any gas
replenishment actions, after 100 million shots the partial
pressures of F.sub.2 and Kr might degrade by between 30% and 100%
and between 0.5% and 5%, respectively.
[0134] To compensate for the various depletion rates of the gases
in the discharge chamber, the present invention performs a variety
of separate and cross-linked gas replenishment procedures, which
take into account the variety of individual degradation rates by
referring to a comprehensive database of different laser operating
conditions. A preferred technique is disclosed in the '653
application already mentioned above. The behavior of the particular
laser in operation and related experiences with gas degradation
under different operating conditions are stored in that database
and are used by a processor-controlled "expert system" to determine
the current conditions in the laser and manage the gas
replenishment or refurbishment operations. A history of gas actions
performed during the current operation of the laser may also be
used in accord with the present invention.
[0135] As mentioned above, series of small gas injections (referred
to as enhanced and ordinary micro gas or halogen injections, or
.mu.HI) can be used to return any constituent gas of an excimer or
molecular laser, particularly the very active halogen, to its
optimal concentration in the discharge chamber without disturbing
significant output beam parameters. However, the gas mixture also
degrades over time as contaminants build up in the discharge
chamber. Therefore, mini gas replacements (MGR) and partial gas
replacements (PGR) are also performed in the preferred methods. Gas
replacement generally involves releasing some gas from the
discharge chamber, including expelling some of the contaminants.
MGR involves replacement of a small amount of gas periodically at
longer intervals than the small .mu.HIs are performed. PGR involves
still larger gas replacement and is performed at still longer
periodic intervals generally for "cleaning" the gas mixture. The
precise intervals in each case depend on consulting current laser
operating conditions and the expert system and comprehensive
database. The intervals are changes of parameters which vary with a
known relationship to the degradation of the gas mixture. As such,
the intervals may be one or a combination of time, pulse count or
variations in driving voltage, pulse shape, pulse duration, pulse
stability, beam profile, coherence, discharge width or bandwidth.
In addition, the accumulated pulse energy dose may used as such an
interval. Each of .mu.HI, MGR and PGR may be performed while the
laser system is up and running, thus not compromising laser
uptime.
[0136] Three exemplary gas replenishment methods for stabilizing an
optimum gas mixture are described below. Many other methods are
possible including combinations of the ones described below. The
methods and parameters used may also be varied during the laser
operation depending on the laser operating conditions and based on
the data base and the expert system. The processor and gas supply
unit are configured to perform many methods based on a
comprehensive database of laser operating conditions and gas
mixtures statuses.
[0137] Each method involves well-defined very small gas actions
with small, successive gas injections preferably by injecting a
premix of less than 10 mbar and more preferably between 0.1 and 2
mbar including a concentration including preferably 5% or less of
the halogen containing species in order not to disturb the laser
operation and output beam parameters. Whatever the composition of
the premix, it is the amount of the halogen in the premix that is
most significant. That is, the preferred amount of the halogen
containing species that is injected in the small gas actions
preferably corresponds to less than 0.1 or 0.2 mbar and more
preferably between 0.001 and 0.02 mbar partial pressure increase in
the laser tube 1.
[0138] The first exemplary gas stabilization method involves
performing gas injections based on operation time. The method takes
into account whether or not the laser is operating, i.e., whether
the laser system is up and performing industrial processing, in
standby mode, or simply shut off. The first method is thus useful
for maintaining either an active or a passive gas composition
status. Time-correlated .mu.HI, MGR and PGR are performed according
to a selectable time interval based on operating conditions. For
example, .mu.HIs may be performed after time intervals t.sub.1,
MGRs after time intervals t.sub.2, and PGRs after time intervals
t.sub.3.
[0139] In accord with the present invention, the time intervals
t.sub.1, t.sub.2 and t.sub.3 are adjusted in real time as are the
amounts and/or compositions of gases injected during the gas
actions. Preferably, the time intervals and gas amounts and
compositions are adjusted from gas action to gas action. In
addition, the driving voltage ranges within which particular gas
actions are performed are preferably also adjusted, at least at
each new fill based on the aging of the tube and optical components
of the laser resonator. Such ranges may be adjusted during
operation, even between new fills, e.g., based on beam-induced
effects on the optical components of the line-narrowing module (see
for a general explanation of such effects U.S. patent application
No. 60/124,804, assigned to the same assignee and hereby
incorporated by reference).
[0140] Below, detailed graphs are described for an operating laser
system in accord with the present invention. Typically, gas actions
occur after several hours if the laser is in the standby-mode
without pulsing or pulsing with low repetition rate (<100 Hz).
If the laser is completely switched off (power-off-mode), a battery
driven internal clock is still running and the expert system can
release an adequate, time controlled number of injections during
the warm-up phase after re-starting the laser. The number and
amount of the injections can be also related to certain driving
voltage start conditions which initiate a preferred sequence of gas
actions to reestablish optimum gas quality.
[0141] FIGS. 15 and 16 are graphs of driving voltage versus time
also illustrating the intervals of periodic .mu.HI and periodic
.mu.HI and MGR, respectively, for a fully operating system in
accord with the present invention. FIG. 15 includes a plot of
driving voltage versus time (A) wherein .mu.HIs are performed about
every 12 minutes, as indicated by the vertical lines (some of which
are designated for reference with a "B") on the graph, for a
narrowband laser running in 2000 Hz burst mode at 10 mJ output beam
energy. The vertical axis only corresponds to graph A. As is shown
by graph A, the small .mu.HIs produce no noticeable discontinuities
in the driving voltage.
[0142] FIG. 16 is a plot (labelled "A") of driving voltage versus
time wherein .mu.HIs are performed about every 12 minutes, as
indicated by the short vertical lines on the graph (again, some of
which are designated for reference with a "B" and the vertical axis
doesn't describe the halogen injections in any way), and MGR is
performed about every 90 minutes, as indicated by the taller
vertical lines on the graph (some of which are designated with a
"C" for reference and again the vertical axis is insignificant in
regard to the MGRs shown), for a narrowband laser running in 2000
Hz burst mode at 10 mJ output beam energy. Again, the driving
voltage is substantially constant around 1.8 KV and no major
changes, e.g., more than %5, are observed.
[0143] A comparison of FIGS. 15 and 16 with FIG. 8 reveals that the
present invention advantageously avoids the conventional approach
which drastically increases the driving voltage as the gas mixture
degrades. By avoiding discontinuities, fluctuations or changes in
the driving voltage in this way, disturbances of meaningful output
beam parameters are also avoided.
[0144] FIG. 17 includes a graph (labelled "A") of pulse energy
stability versus time of the laser pulses by values of standard
deviation (SDEV) and moving average stabilities (.+-.MAV) as
percentages of the absolute pulse energy for a system in accord
with the present invention. The graphs labelled "B" and "C" show
the moving average for groups of 40 pulses each. During this run,
micro-halogen injections were performed resulting in very stable
continuous laser operation without any detectable deviations caused
by the gas replenishment actions.
[0145] The second exemplary gas stabilization method involves
performing gas injections based on shot or pulse count using a shot
or pulse counter. After certain numbers of laser pulses, e.g.,
N(.mu.HI), N(MGR), and N(PGR), depending again on the mode of
operation of the laser, .mu.HI, MGR and PGR can be respectively
performed. Typically, the .mu.HIs amount to about 0.5 . . . 2.0
mbar of fluorine premix (e.g., 1-5% F.sub.2:95-99% Ne) for the KrF,
ArF, XeF or F.sub.2 lasers (Ne being replaceable with He or a mix
of He and Ne) or HCl premix (e.g., 1-5% HCl:1% in Ne or He) for
XeCl or KrCl laser and are released after several hundred thousand
or even after millions of laser shots. Each .mu.HI just compensates
the halogen depletion since the last gas action and typically
corresponds to less than 0.1 mbar of the halogen containing species
and more preferably between 0.001 and 0.02 mbar partial pressure
increase in the laser tube 1 per, e.g., 1 million shots. The actual
amounts and shot intervals vary depending on the type of laser, the
composition of the discharge chamber, the original gas mixture
composition and operating mode, e.g., energy, or repetition rate,
being used.
[0146] A third exemplary method is similar to those described above
using time or pulse count, and this method instead uses accumulated
energy applied to the discharge. Use of this parameter, and
advantages thereof, are set forth in the '525 application. The
total input electrical energy to the discharge is maintained in a
counter for that purpose, and gas actions are performed after
certain intervals or amounts of this input electrical energy are
applied.
[0147] Also, in accord with a preferred embodiment, the intervals
of any of the exemplary methods are dynamically adjusted from
injection to injection, as are the amounts of halogen injected with
each gas action. The interval between the current and next
injection is set based on any one or a combination of parameters
such as the driving voltage or any of the output beam parameters
described above. In addition, the amount of halogen injected in the
current injection and/or the interval between the previous and
current injection may be taken into account.
[0148] The amount of halogen injected in any .mu.HI or enhanced
.mu.HI may be determined in accord with the present invention by
measuring the pressure in the accumulator (see FIGS. 13b and 14a)
and the laser tube at the time of the injection, and/or just
before, and/or just after the injection. The temperatures of the
gases in the accumulator and tube may be measured as well. The
interior volumes of the tube and accumulator are known in advance.
The well-known formula PV=Nk.sub.BT is used to calculate the amount
of halogen injected into the tube during any injection.
[0149] For example, if the accumulator has a measured halogen
partial pressure P.sub.a, and temperature T.sub.a, and a volume
V.sub.a, then the accumulator contains N.sub.a fluorine molecules.
If all of the N.sub.a molecules are injected into the laser tube
during the injection, and the tube has a temperature T.sub.T and
volume V.sub.T, then the change in fluorine partial pressure in the
tube as a result of the injection will be
.DELTA.P(F.sub.2).sub.T=P.sub.aV.sub.aT.sub.T/V.sub.TT.sub.a. Since
it is desired to maintain the total number of fluorine molecules in
the tube, then it may be more appropriate to calculate the change
in the number of fluorine molecules in the tube, i.e.,
.DELTA.N(F.sub.2).sub.T=)P(F.sub.2).sub.TV.sub.T/k.sub.BT.sub.T,
and keep track of that quantity. Then, the amount of halogen and/or
the interval before the next injection is determined based on the
calculated amount of halogen that was injected in the previous
injection, the partial pressure of the halogen in the tube after
the previous injection and/or the amount of halogen that it is
desired to have in the tube after the next injection.
[0150] The overall calculation depends also on the amount of
depletion that the halogen gas has undergone (or will undergo)
between injections. Such depletion is, in principal, known as a
function of many factors, e.g., including time and pulse count (and
possibly any of the parameters enumerated above or others). For
example, a change in halogen partial pressure (or, alternatively,
the number of halogen molecules) in the laser tube in the interval
between injections can be calculated to depend on
k.sub.t.times..DELTA.t and on k.sub.p.times..DELTA.p, wherein
k.sub.t and k.sub.p are constants that depend on the rate of
halogen depletion with time and pulse count, respectively, and
.DELTA.t and .DELTA.p are the amount of time and the number of
pulses, respectively, in the interval under consideration. The
number of pulses .DELTA.p itself depends on the repetition rate,
taking into account also the number of pulses in a burst and the
pause intervals between bursts for a laser operating in burst mode.
Again, other parameters may have an effect and may be additive
terms included with this calculation.
[0151] Now, from one interval to the next, a calculation could be
performed as follows. The increase (or decrease reflected as a
negative sum) in fluorine partial pressure in the laser tube over
the interval is calculated to be:
.DELTA.P(F.sub.2).sub.interval.apprxeq.P(F.sub.2).sub.T
injection-k.sub.t.times..DELTA.t-k.sub.p.times..DELTA.p. Again,
since it is the total number of fluorine molecules that it is
desired to keep constant, then a calculation of the change in the
number of molecules is calculated as:
.DELTA.N(F.sub.2).sub.interval.apprxeq.N(F.sub.2).sub.T
injection-k.sub.t.times..DELTA.t-k.sub.p.times..DELTA.p, where the
constants k.sub.t and k.sub.p would differ from the partial
pressure calculation by a units conversion.
[0152] The overall algorithm would seek to maintain the total
number of halogen molecules (or halogen partial pressure) constant.
Thus, the changes in particle number (or partial pressure) would be
summed continuously over many intervals, or preferably all
intervals since the last new fill. That overall sum would be
maintained as close as possible to zero, in accord with the present
invention.
[0153] As discussed, the shot counter can also be used in
combination with time related gas replenishment, and either of the
shot counter or time related gas replenishment can be used in
combination with the total energy applied to the discharge. The
shot counter or total applied energy can be used for different
laser pulse operation modes, e.g., burst patterns, or continuous
pulsing modes at different pulse repetitions wherein a number of
individual shot or input energy counters N.sub.i(HI) are used. All
of these different counters can be stored in the data base of the
expert system. Which of the different counters N.sub.i(HI) is to be
used at any time is determined by the software of the expert
system.
[0154] FIG. 18 illustrates qualitatively a driving voltage free of
discontinuities when small partial pressure increases are effected
in the laser discharge chamber due to .mu.HIs in accord with the
present invention. The driving voltage is shown as being
substantially constant at around 1.7 KV over 150 million pulses,
while .mu.HIs are performed about once every 12 million pulses. The
pulse energy is also maintained at a constant level.
[0155] A comparison of FIG. 18 with the driving voltage graph of
FIG. 12 shows an advantage of the present invention. In FIG. 12 the
driving voltage is observed to increase steadily until a halogen
injection (HI) is performed, and is then observed to drop
precipitously when the halogen is injected in a large amount in
accord with conventional gas replenishment. These disturbances in
the driving voltage curve of FIG. 12 occur because the intervals
for the HIs are too large and the amounts of halogen injected are
thus too large to prevent the disturbances. As can be deduced from
FIGS. 9-11, these large driving voltage disturbances undesirable
affect meaningful output beam parameters. FIG. 18, on the other
hand, shows no fluctuations in the driving voltage in response to
micro-halogen injections performed in accord with the present
invention.
[0156] FIG. 19 is a graph including two plots. The first plot
following the darkened triangles and labeled "convention HI" is the
energy stability variation versus pulse count for a system using a
conventional HI algorithm and shows sharp discontinuities in the
energy stability. For example, the first HI is shown to produce a
leap from 0.95% to 1.10% almost instantaneously in response to the
HI. The second plot following the darkened circles and labeled
".mu.HI-present invention" is the energy stability variation versus
pulse count for a system using a .mu.HI algorithm in accord with
the present invention wherein discontinuities are substantially
minimized in the energy stability.
[0157] FIG. 20 is a graph also including two plots. The first plot
following the darkened triangles and labeled "conventional HI" is
the beam divergence versus pulse count for a system using a
conventional HI algorithm and shows sharp discontinuities in the
beam divergence. For example, the first HI is shown to produce a
sharp drop from 1.175 mrad to 1.125 mrad almost instantaneously in
response to the HI. The second plot following the darkened circles
and labeled ".mu.HI-present invention" is the beam divergence
versus pulse count for a system using a .mu.HI algorithm in accord
with the present invention wherein discontinuities are
substantially minimized in the beam divergence.
[0158] The expert system can use a different kind of shot counter,
e.g., N(MGR) and/or N(PGR) for other types of gas actions (i.e.,
different from the N(.mu.HI)). MGR and PGR replace or substitute
different gases of the gas mixture in the laser tube by
predetermined amounts. As mentioned, MGR and PGR include a gas
injection accompanied by a release of gases from the laser tube,
whereas .mu.HIs do not involve a release of gases. Gas releases can
be performed simply to reduce the pressure in the laser tube, as
well as for expelling contaminants from the gas mixture. Unequal
degradations of the individual gas components within the gas
mixture are nicely compensated using MGR and PGR, and again,
different numbers N.sub.i(MGR) and N.sub.i(PGR) may be used for
different operating modes and conditions as determined by the
expert system. All of these settings, i.e., N.sub.i(.mu.HI),
N.sub.i(MGR), N.sub.i(PGR) and the separately selectable portions
of injections for each gas can be adapted for the aging of the
laser tube, and/or the aging of the resonator optics, taking into
account changing conditions of gas consumption and replenishments
as the laser system components age. The amount of compensation can
be pre-selected by manual settings or based on settings in the data
base of the computer controlled expert system. For MGR, like
.mu.HI, the portions of injected gases amount to a few mbar total
pressure increase in the laser tube (or percent only). The MGR is
combined with a small pressure release of some few to 10 mbar of
the pressure of the tube, preferably bringing the pressure in the
tube back near to the pressure in the tube just after the last new
fill.
[0159] More than one gas may be injected or replaced in the same
gas action. For example, a certain amount of halogen and a certain
amount of an active rare gas and/or a gas additive for an excimer
laser may be injected together into the laser tube. This injection
may be accompanied by a small pressure release as with MGR.
Alternatively, this mixture of the halogen and rare or additive
gases may simply be injected to increase the partial pressure of
each gas within the discharge chamber without any accompanying
release of gases.
[0160] A further exemplary gas stabilization method involves
performing gas injections based on operating driving voltage values
of the laser. This method can be and preferably is advantageously
combined with any of the first, second and third exemplary methods.
That is, the time related t.sub.1(.mu.HI), t.sub.2(MGR),
t.sub.3(PGR) and the pulse and/or input electrical energy to the
discharge counter-related N.sub.i(.mu.HI), N.sub.i(MGR),
N.sub.i(PGR) gas actions, discussed above, are generally adjusted
during operation depending on the value of the operating driving
voltage, and preferably, on the operation band of the driving
voltage.
[0161] Referring to FIG. 21, several driving voltage levels
(HV.sub.i) can be defined wherein particular gas actions are
predetermined to be performed. The processor monitors the driving
voltage and causes the gas supply unit to perform gas injections of
varying degrees and partial and mini gas replacements of varying
degrees depending on the value of the driving voltage, or which
preset range the current operating driving voltage is in (y-axis of
FIG. 21), based on such parameters as time, pulse count and/or
total input electrical energy to the discharge, etc. (see '525
application mentioned above) x-axis of FIG. 21).
[0162] An example in accord with the present invention is next
described with reference to FIG. 21. The laser system may operate
at driving voltages between HV.sub.min and HV.sub.max. The actual
operating minimum and maximum driving voltages are set to be in a
much smaller range between HV.sub.1 and HV.sub.6, as illustrated by
the broken ordinate axis. An advantage of the present invention is
that the range HV.sub.1 to HV.sub.6 itself may be reduced to a very
small window such that the operating voltage is never varied
greatly during operation of the laser. Where this operating range
itself lies between HV.sub.min and HV.sub.max, i.e., the actual
voltage range (in Volts) corresponding to the range may be
adjusted, e.g., to increase the lifetimes of the optical components
of the resonator and the laser tube, e.g., such as by adjusting an
output energy attenuating gas additive (see the '126
application).
[0163] The coordinate axis of FIG. 21 denotes the gas actions that
are performed, based on one or more accumulated parameters, when
the driving voltage is in each interval. The general order of
performance of the gas actions is from left to right as the gas
mixture ages. However, when each gas action is performed, the
driving voltage is checked, and the next gas action may that
corresponding to the same driving voltage range, or a different one
denoted to the left or the right of that range. For example, after
a PGR is performed (when it is determined that the driving voltage
is above HV.sub.5), the driving voltage may be reduced to between
HV.sub.2 and HV.sub.3, and so the system would return to ordinary
.mu.HI and MGR.sub.1 gas control operations.
[0164] Within the operating range between HV.sub.1 and HV.sub.6,
several other ranges are defined. For example, when the driving
voltage HV is between HV.sub.1 and HV.sub.2 (i.e.,
HV.sub.1<HV<HV.sub.2), no gas actions are performed as there
is a sufficient amount of halogen in the gas mixture. When the
driving voltage is between HV.sub.2 and HV.sub.3 (i.e.,
HV.sub.2<HV<HV.sub.3), MGR.sub.1 and ordinary .mu.HI are
performed periodically based on the accumulated parameter(s) (i.e.,
input electrical energy to the discharge, time, and/or pulse count,
etc.). This is the ordinary range of operation of the system in
accord with the present invention.
[0165] When the driving voltage is between HV.sub.3 and HV.sub.4
(i.e., HV.sub.3>HV>HV.sub.4), one or both of the injection
amounts of the .mu.HIs and the MGRs with corresponding gas releases
is increased. In this example, only the .mu.HIs are increased.
Thus, the range between HV.sub.3 and HV.sub.4 in FIG. 21 is the
range within which enhanced .mu.HIs are performed, and the same MGR
amounts as in the previous range between HV.sub.2 and HV.sub.3 are
maintained.
[0166] Enhanced .mu.HIs may differ from ordinary .mu.HIs in one or
both of two ways. First, the amount per injection may be increased.
Second, the interval between successive .mu.HIs may be
increased.
[0167] The range between HV.sub.4 and HV.sub.5 (i.e.,
HV.sub.4<HV<HV.sub.5) represents a new range within which one
or both of the injection amounts of the pHIs and the MGRs with
corresponding gas releases is increased. In this example, only the
MGRs are increased as compared with the range HV.sub.3 to HV.sub.4.
Thus, an enhanced amount of halogen gas is injected (with
corresponding release of gases) during each MGR.sub.2 than the
ordinary amount MGR.sub.1 when the driving voltage is in the range
between HV.sub.4 and HV.sub.5. Alternatively, or in combination
with replacing the gas in larger amounts, the mini gas replacements
MGR.sub.2 are performed at smaller intervals than the MGR.sub.1 are
performed. In each of the preferred and alternative MGR.sub.2
procedures, the contaminants in the discharge chamber are reduced
at smaller intervals (e.g., of accumulated input energy to the
discharge, pulse count and/or time, among others) compared with the
MGR.sub.1 procedures that are performed at the lower driving
voltage range between HV.sub.3 and HV.sub.4. The .mu.HIs are also
preferably performed periodically in this range to recondition the
gas mixture. It is noted here that several ranges wherein either or
both of the amounts injected during the .mu.HIs and MGRs is
adjusted may be defined each corresponding to a defined driving
voltage range. Also, as mentioned above with respect to monitoring
the pressure (and optionally the temperature) in the accumulator
(and optionally the laser tube), the amount injected may be
adjusted for each injection.
[0168] When the driving voltage is above HV.sub.5 (i.e.,
HV.sub.5<HV<HV.sub.6), a still greater gas replacement PGR is
implemented. PGR may be used to replace up to ten percent or more
of the gas mixture. Certain safeguards may be used here to prevent
unwanted gas actions from occurring when, for example, the laser is
being tuned. One is to allow a certain time to pass (such as
several minutes) after the HV.sub.5 level is crossed before the gas
action is allowed to be performed, thus ensuring that the driving
voltage actually increased due to gas mixture degradation. When the
driving voltage goes above HV.sub.6, then it is time for a new fill
of the laser tube. It is noted here that the magnitudes of the
driving voltages ranges shown in FIG. 21 are not necessarily drawn
to scale.
[0169] FIG. 22 is a flow diagram for performing ordinary and
enhanced .mu.HIs, MGRs and PGRs in accord with the present
invention and the example set forth as FIG. 21. The procedure
starts with a new fill, wherein the discharge chamber is filled
with an optimal gas mixture. The laser can thereafter be in
operation for industrial applications, in stand-by mode or shut off
completely. A driving voltage check (HV-check) is performed after
the current driving voltage (HV) is measured.
[0170] The measured driving voltage (HV) is compared with
predetermined values for HV.sub.1 through HV.sub.6. The processor
determines whether HV lies between HV.sub.1 and HV.sub.2 (i.e.,
HV.sub.1<HV<HV.sub.2) and thus path (1) is followed and no
gas actions are to be performed and the procedure returns to the
previous step. Although not shown, if the HV lies below HV.sub.1,
then a procedure may be followed to decrease the halogen
concentration in the laser tube, such as by releasing some laser
gas and/or injecting some buffer gas from/into the laser tube.
[0171] If the processor determines that the HV lies between
HV.sub.2 and HV.sub.3, then the system is within the ordinary
operating driving voltage band. If it is within the ordinary
operating band, then path (2) is followed whereby ordinary .mu.HIs
and MGR.sub.1 may be performed based preferably on time, input
electrical energy to the discharge and/or pulse count intervals as
predetermined by the expert system based on operating conditions.
Again each gas action may be adjusted depending on the calculated
partial pressure or number of halogen molecules in the laser tube,
as described above.
[0172] The .mu.HIs and MGR.sub.1 performed when path (2) is
followed may be determined in accordance with any method set forth
in U.S. patent application Ser. No. 09/167,653, already
incorporated by reference. If HV is not within the ordinary
operating band, then it is determined whether HV lies below
HV.sub.2 (i.e., HV<HV.sub.2). If HV is below HV.sub.2, then path
(2) is followed and no gas actions are performed.
[0173] If HV lies between HV.sub.3 and HV.sub.4 (i.e.,
HV.sub.3<HV<HV.sub.4), then path (3) is followed and enhanced
.mu.HI and MGR.sub.1 may be performed again based on the value or
values of the time, pulse count and/or applied electrical energy to
the discharge counter(s) being used. The precise amounts and
compositions of gases that are injected and those that are released
are preferably determined by the expert system and will depend on
operating conditions.
[0174] If HV lies between HV.sub.4 and HV.sub.5 (i.e.,
HV.sub.4<HV<HV.sub.5), then path (4) is followed and enhanced
.mu.HI and MGR.sub.2 may be performed depending on checking the
values of the counters. Again, the precise amounts and compositions
of gases that are injected and those that are released are
preferably determined by the expert system and will depend on
operating conditions.
[0175] If HV lies between HV.sub.5 and HV.sub.6 (i.e.,
HV.sub.5<HV<HV.sub.6), then PGR is performed. If HV lies
above HV.sub.6 (i.e., HV.sub.6<HV), then a new fill is
performed.
[0176] After any of paths (2)-(5) is followed and the corresponding
gas actions are performed, and preferably after a specific settling
time, the method returns to the step of determining the operating
mode of the laser and measuring and comparing HV again with the
predetermined HV levels HV.sub.1 through HV.sub.6.
[0177] The setting of all of these different driving voltage levels
and time, applied electrical energy to the discharge and/or pulse
count schedules can be done individually or can refer to the
computer controlled data base where they are stored for different
operation conditions. The operation of the laser at different
HV-levels under different operation conditions such as continuous
pulsing or burst mode may be taken into consideration.
[0178] In another preferred embodiment, a partial new fill
procedure may be performed according to FIGS. 23 and 24. As shown
in FIG. 23, an additional HV range is established which lies above
the PGR range 5 and yet below an additional threshold value
HV.sub.7. The remainder of the graph shown in FIG. 23 is preferably
the same as that shown in FIG. 21, and the discussion of the other
ranges 1-5 will not be repeated here.
[0179] When the processor determines that the high voltage is above
HV.sub.6, then either a new fill or a partial new fill will be
performed depending on whether the high voltage is at or below
HV.sub.7 wherein a partial new fill is to be performed, or is above
HV.sub.7, wherein a total new fill is performed. When a total new
fill is performed, substantially 100% of the gas mixture is emptied
from the discharge chamber and a totally fresh gas mixture is
introduced into the laser chamber. However, when a partial new fill
is performed, only a fraction (5% to 70% or around 0.15 bar to 2
bar, as examples) of the total gas mixture is released. More
particularly preferred amounts would be between 20% and 50% or 0.6
bar to 1.5 bar. A specifically preferred amount would around 1 bar
or around 30% of the gas mixture. Experiments have shown that
implementing a partial new fill procedure wherein 1 bar is
exchanged increases the gas lifetime by as much as five times over
now having the procedure.
[0180] The amount that may be released is an amount up to which a
substantial duration of time is used to get the gas out with a
pump, and so the amount may be more than 50%, and yet may take
substantially less time than a total new fill. Thus, a partial new
fill procedure has the advantage that a large amount of aged gas is
exchanged with fresh gas in a short amount of time, thus increasing
wafer throughput when the laser is being used in lithographic
applications, for example.
[0181] Referring now to FIG. 24, the flow chart is the same as that
shown in FIG. 22, except that when the processor determines that
the high voltage is above HV.sub.6, an additional determination is
made whether the high voltage is at or below HV.sub.7. If the
answer is yes, i.e., that the high voltage is at below HV.sub.7,
then a partial new fill is initiated, whereby less than
substantially 100% of the gas mixture is taken out of the discharge
chamber and replaced with fresh gas. Advantageously, the system is
only taken offline for a short time compared with performing a
total new fill. If the answer is no, i.e., that the high voltage is
above HV.sub.7, then a new fill is performed just as described
above with respect to FIG. 22. As mentioned, experiments have shown
that the gas lifetime can be improved by as much as five times
before the new fill range would be reached when the partial new
fill procedure is implemented.
[0182] It is to be understood that a system not using all of the
ranges 1-6 and the new fill/partial new fill procedures of range 6
may be advantageously implemented. For example, in FIG. 23, a
system that uses only a single one of the ranges with the partial
new fill and new fill may be used, and the gas lifetime improved.
With some ranges removed, the partial new fill range may be moved
to a lower threshold high voltage. Even according to FIG. 21, one
or more of the ranges may not be implemented, and the system may
still improve the performance of the laser system. It is preferred
that all of the ranges and corresponding gas actions be used for
optimum laser system performance.
[0183] The combination of all of these different kinds of gas
control and replenishment mechanisms involves harmonizing many
factors and variables. Combined with the expert system and
database, the processor controlled laser system of the present
invention offers an extended gas lifetime before a new fill is
necessary. In principle, bringing down the laser system for new
fill might be totally prevented. The lifetime of the laser system
would then depend on scheduled maintenance intervals determined by
other laser components such as those for laser tube window or other
optical components exchange. Again, as mentioned above with
reference to the '126 application, even the lifetimes of the laser
tube and resonator components may be increased to increase the
intervals between downtime periods.
[0184] The above described gas replenishment procedures may be
combined with cryogenic or electrostatic gas purification
techniques, whereby contaminants such as rare gas fluorides, i.e.,
AF.sub.n molecules, where A=Kr, Ar or Xe and n=2, 4 or 6) or other
contaminants as mentioned above are removed from the gas mixture.
For this purpose, U.S. Pat. Nos. 4,534,034, 5,001,721, 5,111,473,
5,136,605 and 5,430,752 are hereby incorporated by reference into
the present application. Standard methods typically include using a
cold trap to freeze out contaminants before recycling the gas back
into the discharge chamber. Some of the contaminants being frozen
out are molecular combinations of active gases such as the active
rare and halogen gases of excimer lasers. Thus, a significant
amount of these important laser gases is removed from gas mixture
in the discharge chamber. The result is a rapid decrease in rare
and halogen gas concentrations undesirably affecting output beam
parameters.
[0185] In summary, the present invention provides a method and
procedure for stabilizing an original or optimal gas composition of
a gas discharge laser, and particularly an excimer or molecular
fluorine (F.sub.2) laser. During a long period of operation of the
laser in a running or stand-by mode, the depletion of the laser gas
is continuously monitored by monitoring and controlling the high
voltage, laser pulse shape, ASE, elapsed time after new fill or
other additional laser parameters some of which have been set forth
above, in addition to accumulated electrical energy applied to the
discharge, time and/or pulse count. According to a database of
known histories and trends of key operating parameters for various
lasers operating under various conditions, a processor controlled
procedure is applied to replenish the gas degradation. The
stabilization process involves using a number of tiny gas actions
(micro injections) performed preferably based on specified time,
driving voltage change, input electrical energy to the discharge
and/or shot count intervals, a combination thereof or some other
interval relating to a parameter which changes with a known
relationship to the gas mixture degradation. A careful combination
of .mu.HIs and MGRs of various amounts, and PGRs, are used to
effect very nearly complete stabilization of the laser gas mixture
over a potentially unlimited duration. Most importantly, the gas
actions described herein do not disturb meaningful output beam
parameters or operation of the laser, because they are smooth and
controlled based on an expert system comprising myriad operating
conditions of the laser system. Thus, the laser can operate without
interruption during the gas replenishment actions and industrial
processing can be performed with high efficiency.
[0186] A method is provided below for maintaining an output energy
of an excimer or molecular fluorine laser within a tolerance range
of a desired energy and for maintaining an input driving voltage
within a tolerance range of an optimal input driving voltage, the
excimer or molecular fluorine laser having a laser tube filled with
a gas mixture, multiple electrodes within the laser tube coupled
with a gas discharge circuit for energizing the gas mixture and a
resonator for generating a laser beam. The method includes
operating the laser to emit the laser beam within the tolerance
range of the desired energy. An energy of the laser beam is
measured. An input driving voltage is adjusted to maintain the
energy of the laser beam within the tolerance range of the desired
energy. A value of the input driving voltage is determined. A total
pressure of the gas mixture within the laser tube is adjusted to
maintain the input driving voltage within said tolerance range of
said optimal input driving voltage.
[0187] A method is further provided for energy stabilization of an
excimer or molecular fluorine laser having a laser tube filled with
a gas mixture, multiple electrodes within the laser tube coupled
with a gas discharge circuit for energizing the gas mixture and a
resonator for generating a laser beam. The method includes
operating the laser to emit the laser beam. An energy of the laser
beam is measured as a first energy. A step of determining whether
the first energy lies within a tolerance range of a desired energy
is performed. A pressure of the gas mixture within the laser tube
is adjusted to adjust the energy of the laser beam when the
measured first energy is determined to be outside the tolerance
range of the desired energy.
[0188] A method is further provided for optimizing performance of
an excimer or molecular fluorine laser system having a laser tube
filled with a gas mixture, multiple electrodes within the laser
tube coupled with a gas discharge circuit for energizing the gas
mixture and a resonator for generating a laser beam. The method
includes setting a value of input driving voltage within an input
driving voltage range which includes an optimum driving voltage
predetermined to provide optimum laser system performance. A new
fill of the gas mixture within the laser tube is performed. The
laser is operated at the set value of input driving voltage to emit
the laser beam. An energy of the laser beam is measured. A step of
determining whether the measured energy is within a tolerance range
of a desired energy is performed. A pressure of the gas mixture
within the laser tube is adjusted to adjust the energy of the laser
beam when the measured energy is determined to be outside the
tolerance range of the desired energy.
[0189] A method is further provided for optimizing performance of
an excimer or molecular fluorine laser system having a laser tube
filled with a gas mixture, multiple electrodes within the laser
tube coupled with a gas discharge circuit for energizing the gas
mixture and a resonator for generating a laser beam. The method
includes setting a value of input driving voltage within a
tolerance input driving voltage range which includes an optimum
driving voltage predetermined to provide optimum laser system
performance. A new fill of the gas mixture within the laser tube is
performed. The laser is operated to emit the laser beam at a
desired energy. A step of determining whether the input driving
voltage applied to achieve the desired energy is within the input
driving voltage range is performed. A pressure of the gas mixture
within the laser tube is adjusted to adjust the input driving
voltage to be applied to achieve the desired energy when the input
driving voltage is determined to be outside the tolerance input
driving voltage range.
[0190] An excimer or molecular fluorine laser system is provided
including a laser tube filled with a gas mixture, multiple
electrodes within the laser tube connected to a power supply
circuit for providing a driving voltage to the electrodes to
energize the gas mixture, a resonator for generating a laser beam
at a desired energy, a gas exchange unit connected to the laser
tube including an auxiliary volume at a pressure below a pressure
of the gas mixture within the laser tube, and a processor for
controlling gaseous flow between the gas exchange unit and the
laser tube. The gas exchange unit and the processor are configured
to permit amounts of the gas mixture to release into the auxiliary
volume to reduce the pressure within the laser tube to increase the
driving voltage applied to achieve the desired energy when the
driving voltage is determined to be below a tolerance driving
voltage range to achieve the desired energy.
[0191] An excimer or molecular fluorine laser system is further
provided including a laser tube filled with a gas mixture, multiple
electrodes within the laser tube connected to a power supply
circuit for providing a driving voltage to the electrodes to
energize the gas mixture, a resonator for generating a laser beam
at a desired energy, a gas exchange unit connected to the laser
tube, and a processor for controlling gaseous flow between the gas
exchange unit and the laser tube. The gas exchange unit and the
processor are configured to permit amounts of gas to be added to
the laser tube to increase the pressure within the laser tube to
reduce the driving voltage applied to achieve the desired energy
when the driving voltage is determined to be above a tolerance
driving voltage range to achieve the desired energy.
[0192] An excimer or molecular fluorine laser, and method of
operating thereof, is provided below in accord with a preferred
embodiment. The average energy of output pulses or the energy dose
is stabilized around a predetermined substantially constant value
by monitoring the average pulse energy or energy dose and adjusting
the total pressure of the laser gas mixture in the laser tube when
the pulse energy or energy dose is varied from the predetermined
constant value, preferably in a feedback arrangement involving a
processor, an energy detector and a gas control unit. In this way,
an applied driving discharge voltage from a pulser module to
electrodes within a laser tube or discharge chamber of the laser is
maintained within a predetermined voltage range which is smaller
than in conventional laser systems. In addition, the average pulse
energy or energy dose is maintained around the predetermined
constant average energy and/or energy dose, while preferably a
duration between new fills is not reduced and more preferably such
duration is actually increased.
[0193] It is noted here that the laser tube is the portion of the
laser system that is filled with the gas mixture that is excited to
produce the laser beam, while the discharge chamber may be referred
to as a portion of the laser tube which also includes a heat
exchanger and a fan. However, the terms laser tube and discharge
chamber are meant each to refer to the laser tube whenever either
term is used herein. It is further noted here that when the term
energy is used herein, the energy may non-exhaustively refer to an
average pulse energy over several pulses, an energy dose at a
workpiece, a pulse energy, an average power of a pulsed output
laser beam.
[0194] An excimer or molecular fluorine laser system according to a
preferred embodiment is configured with a processor for controlling
one or more special gas procedures for the operation of the laser
within a certain limited range of driving high voltages, i.e.,
nearly HV for an optimal laser operation (HV.sub.opt), independent
of the age of the laser tube and of optical components of the laser
resonator. Such algorithms include adjustments of the total
pressure within the laser tube. In addition, an algorithm is
provided for calculating an amount of pressure adjustment before
the adjustment is performed, for providing greater precision
pressure in the adjustment. Further algorithms are provided for
including pressure adjustments in energy control algorithms of an
expert system of the laser, including measuring an output laser
energy to be outside a deviation tolerance, either above or below a
desired average pulse energy or energy dose, and initiating a
pressure release or a pressure add, respectively, to the total
pressure of the laser tube.
[0195] The average pulse energy or energy dose is preferably
stabilized with the pulser circuit of the laser configured to apply
a driving voltage to the electrodes at or near HV.sub.opt
notwithstanding an aging state of the laser tube or resonator
optics by adjusting the total gas pressure in the laser tube
following a new fill and during the laser operation such that
application of HV.sub.opt to the discharge of the laser results in
generation of a laser beam of energy at or near a desired average
pulse energy or energy dose. Therefore, an excimer or molecular
fluorine laser system is provided wherein the average pulse energy
may be adjusted by adjusting another parameter of the laser system
other than the input driving voltage, which permits the input
driving voltage to operate within a narrower range of voltages than
conventional systems without reducing a duration between new
fills.
[0196] FIG. 25 schematically shows an excimer or molecular fluorine
laser system according to a preferred embodiment. The preferred gas
discharge laser system is a DUV or VUV laser system, such as an
excimer laser system, e.g., ArF or KrF, or a molecular fluorine
(F2) laser system for use with a deep ultraviolet (DUV) or vacuum
ultraviolet (VUV) lithography system, is schematically shown.
Alternative configurations for laser systems for use in such other
industrial applications as TFT annealing, photoablation and/or
micromachining, e.g., are understood by those skilled in the art as
being similar to and/or modified from the system shown in FIG. 25
to meet the requirements of that application. For this purpose,
alternative DUV or VUV laser system and component configurations
are described at U.S. patent application Ser. Nos. 09/317,695,
09/130,277, 09/244,554, 09/452,353, 09/317,527, 09/343,333,
09/512,417, 09/599,130, 60/162,735, 60/166,952, 60/171,172,
60/141,678, 60/173,993, 60/166,967, 60/147,219, 60/170,342,
60/162,735, 60/178,445, 60/166,277, 60/167,835, 60/171,919,
60/202,564, 60/204,095, 60/172,674, 09/574,921, 09/734,459,
09/741,465, 09/686,483, 09/715,803, and 60/181,156, and U.S. Pat.
Nos. 6,005,880, 6,061,382, 6,020,723, 5,946,337, 6,014,206,
6,157,662, 6,154,470, 6,160,831, 6,160,832, 5,559,816, 4,611,270,
5,761,236, each of which is assigned to the same assignee as the
present application and is hereby incorporated by reference.
[0197] The system shown in FIG. 25 generally includes a laser
chamber 102 (or laser tube including a heat exchanger and fan for
circulating a gas mixture within the chamber 102 or tube) having a
pair of main discharge electrodes 103 connected with a solid-state
pulser module 4, and a gas handling module 106. The gas handling
module 106 has a valve connection to the laser chamber 102 so that
halogen, rare and buffer gases, and preferably a gas additive, may
be injected or filled into the laser chamber, preferably in
premixed forms (see U.S. patent application Ser. No. 09/513,025,
which is assigned to the same assignee as the present application,
and U.S. Pat. No. 4,977,573, which are each hereby incorporated by
reference) for ArF and KrF excimer lasers, and halogen and buffer
gases, and any gas additive, for the F.sub.2 laser. The solid-state
pulser module 104 is powered by a high voltage power supply 108.
The laser chamber 102 is surrounded by optics module 110 and optics
module 112, forming a resonator. The optics modules 110 and 112 may
be controlled by an optics control module (not shown), or may be
alternatively directly controlled by a computer or processor 116,
as shown.
[0198] The processor 116 for laser control receives various inputs
and controls various operating parameters of the system. A
diagnostic module 118 receives and measures one or more parameters,
such as pulse energy, average energy and/or power, and preferably
wavelength, of a split off portion of the main beam 120 via optics
for deflecting a small portion of the beam toward the module 118,
such as preferably a beam splitter module (not specifically shown).
The beam 120 is preferably the laser output to an imaging system
(not shown) and ultimately to a workpiece (also not shown) such as
for lithographic applications, and may be output directly to an
application process. The laser control computer 116 communicates
through an interface 124 with a stepper/scanner computer, other
control units and/or other external systems (not otherwise
shown).
[0199] The laser chamber 102 contains a laser gas mixture and
includes one or more preionization electrodes (not shown) in
addition to the pair of main discharge electrodes 103. Preferred
main electrodes 103 are described at U.S. patent application Ser.
Nos. 09/453,670 and 09/791,430, each of which is assigned to the
same assignee as the present application and is hereby incorporated
by reference. Other electrode configurations are set forth at U.S.
Pat. Nos. 5,729,565 and 4,860,300, each of which is assigned to the
same assignee, and alternative embodiments are set forth at U.S.
Pat. Nos. 4,691,322, 5,535,233 and 5,557,629, all of which are
hereby incorporated by reference. Preferred preionization units are
set forth at U.S. patent application Ser. Nos. 09/692,265,
09/532,276, and 09/247,887, each of which is assigned to the same
assignee as the present application, and alternative embodiments
are set forth at U.S. Pat. Nos. 5,337,330, 5,818,865 and 5,991,324,
all of the above patents and patent applications being hereby
incorporated by reference.
[0200] The solid-state pulser module 104 and high voltage power
supply 108 supply electrical energy in compressed electrical pulses
to the preionization and main electrodes 103 within the laser
chamber 102 to energize the gas mixture. Components of the
preferred pulser module and high voltage power supply may be
described at U.S. patent application Ser. Nos. 09/640,595,
09/838,715, 09/858,147, 09/432,348 and 09/390,146, and 09/858,147,
and U.S. Pat. Nos. 6,005,880 and 6,020,723, each of which is
assigned to the same assignee as the present application and which
is hereby incorporated by reference into the present application.
Other alternative pulser modules are described at U.S. Pat. Nos.
5,982,800, 5,982,795, 5,940,421, 5,914,974, 5,949,806, 5,936,988,
6,028,872, 6,151,346 and 5,729,562, each of which is hereby
incorporated by reference. A conventional pulser module may
generate electrical pulses in excess of 1-3 Joules of electrical
power (see the '988 patent, mentioned above).
[0201] The laser resonator which surrounds the laser chamber 102
containing the laser gas mixture includes optics module 110
preferably including line-narrowing optics for a line narrowed
excimer or molecular fluorine laser, which may be replaced by a
high reflectivity mirror or the like in a laser system wherein
either line-narrowing is not desired, or if line narrowing is
performed at the front optics module 112, or a spectral filter
external to the resonator is used, or if the line-narrowing optics
are disposed in front of the HR mirror, for narrowing the bandwidth
of the output beam.
[0202] The laser chamber 102 is sealed by windows transparent to
the wavelengths of the emitted laser radiation 120. The windows may
be Brewster windows or may be aligned at another angle, e.g.,
5.degree., to the optical path of the resonating beam. One of the
windows may also serve to output couple the beam.
[0203] After a portion of the output beam 120 passes the outcoupler
of the optics module 112, that output portion preferably impinges
upon a beam splitter module (not shown) which includes optics for
deflecting a portion of the beam to the diagnostic module 118, or
otherwise allowing a small portion of the outcoupled beam to reach
the diagnostic module 118, while a main beam portion 120 is allowed
to continue as the output beam 120 of the laser system (see U.S.
patent application Ser. Nos. 09/771,013, 09/598,552, and 09/712,877
which are assigned to the same assignee as the present invention,
and U.S. Pat. No. 4,611,270, each of which is hereby incorporated
by reference. Preferred optics include a beamsplitter or otherwise
partially reflecting surface optic. The optics may also include a
mirror or beam splitter as a second reflecting optic. More than one
beam splitter and/or HR mirror(s), and/or dichroic mirror(s) may be
used to direct portions of the beam to components of the diagnostic
module 118. A holographic beam sampler, transmission grating,
partially transmissive reflection diffraction grating, grism, prism
or other refractive, dispersive and/or transmissive optic or optics
may also be used to separate a small beam portion from the main
beam 120 for detection at the diagnostic module 118, while allowing
most of the main beam 120 to reach an application process directly
or via an imaging system or otherwise.
[0204] The output beam 120 may be transmitted at the beam splitter
module while a reflected beam portion is directed at the diagnostic
module 118, or the main beam 120 may be reflected, while a small
portion is transmitted to the diagnostic module 118. The portion of
the outcoupled beam which continues past the beam splitter module
is the output beam 120 of the laser, which propagates toward an
industrial or experimental application such as an imaging system
and workpiece for photolithographic applications.
[0205] Particularly for the molecular fluorine laser system, and
for the ArF laser system, an enclosure (not shown) may seal the
beam path of the beam 120 such as to keep the beam path free of
photoabsorbing species. Smaller enclosures may seal the beam path
between the chamber 102 and the optics modules 110 and 112 and
between the beam splitter (not shown, see above) and the diagnostic
module 118. Preferred enclosures are described in detail in U.S.
patent application Ser. Nos. 09/343,333, 09/598,552, 09/594,892 and
09/131,580, which are assigned to the same assignee and are hereby
incorporated by reference, and U.S. Pat. Nos. 5,559,584, 5,221,823,
5,763,855, 5,811,753 and 4,616,908, all of which are hereby
incorporated by reference.
[0206] The diagnostic module 118 preferably includes at least one
energy detector. This detector measures the total energy of the
beam portion that corresponds directly to the energy of the output
beam 120 (see U.S. Pat. No. 4,611,270 and U.S. patent application
Ser. No. 09/379,034, each of which is assigned to the same assignee
and is hereby incorporated by reference). An optical configuration
such as an optical attenuator, e.g., a plate or a coating, or other
optics may be formed on or near the detector or beam splitter
module 121 to control the intensity, spectral distribution and/or
other parameters of the radiation impinging upon the detector (see
U.S. patent application Ser. Nos. 09/172,805, 09/741,465,
09/712,877, 09/771,013 and 09/774,238, each of which is assigned to
the same assignee as the present application and is hereby
incorporated by reference).
[0207] One other component of the diagnostic module 118 is
preferably a wavelength and/or bandwidth detection component such
as a monitor etalon or grating spectrometer (see U.S. patent
application Ser. Nos. 09/416,344, 09/791,496, 09/686,483,
09/791,431, each of which is assigned to the same assignee as the
present application, and U.S. Pat. Nos. 4,905,243, 5,978,391,
5,450,207, 4,926,428, 5,748,346, 5,025,445, and 5,978,394, all of
the above wavelength and/or bandwidth detection and monitoring
components being hereby incorporated by reference.
[0208] Other components of the diagnostic module may include a
pulse shape detector or ASE detector, such as are described at U.S.
patent application Ser. Nos. 09/484,818 and 09/418,052,
respectively, each of which is assigned to the same assignee as the
present application and is hereby incorporated by reference, such
as for gas control and/or output beam energy stabilization, or to
monitor the amount of amplified spontaneous emission (ASE) within
the beam to ensure that the ASE remains below a predetermined
level, as set forth in more detail below. There may be a beam
alignment monitor, e.g., such as is described at U.S. Pat. No.
6,014,206, or beam profile monitor, e.g., U.S. patent application
Ser. No. 09/780,124, which is assigned to the same assignee,
wherein each of these patent documents is hereby incorporated by
reference.
[0209] The processor or control computer 116 receives and processes
values of some of the pulse shape, energy, ASE, energy stability,
energy overshoot for burst mode operation, wavelength, spectral
purity and/or bandwidth, among other input or output parameters of
the laser system and output beam. The processor 116 also controls
the line narrowing module to tune the wavelength and/or bandwidth
or spectral purity, and controls the power supply and pulser module
104 and 108 to control preferably the moving average pulse power or
energy, such that the energy dose at points on the workpiece is
stabilized around a desired value. In addition, the computer 116
controls the gas handling module 106 which includes gas supply
valves connected to various gas sources. Further functions of the
processor 116 such as to provide overshoot control, energy
stability control and/or to monitor input energy to the discharge,
are described in more detail at U.S. patent application Ser. No.
09/588,561, which is assigned to the same assignee and is hereby
incorporated by reference.
[0210] As shown in FIG. 25, the processor preferably communicates
the solid-state pulser module 104 and HV power supply 108,
separately or in combination, the gas handling module 106, the
optics modules 110 and/or 112, the diagnostic module 118, and an
interface 124. The processor preferably also controls an auxiliary
volume 126 which is preferably connected to a vacuum pump 128 for
releasing gases from the laser tube 102 for reducing a total
pressure in the tube 102 according to preferred embodiments set
forth in more detail below. In this way, the processor controls the
input voltage to the discharge 130 and the laser tube total
pressure 132 for controlling the average energy or energy dose in
the laser beam 120 output by the laser in a feedback control loop
including receiving pulse energy and/or beam power data from the
diagnostic module 118, and preferably according to one or more
algorithms set forth below with reference to FIGS. 26-29.
[0211] The laser gas mixture is initially filled into the laser
chamber 102 in a process referred to herein as a "new fills". In
such procedure, the laser tube is evacuated of laser gases and
contaminants, and re-filled with an ideal gas composition of fresh
gas. The gas composition for a very stable excimer or molecular
fluorine laser in accord with the preferred embodiment uses helium
or neon or a mixture of helium and neon as buffer gas(es),
depending on the particular laser being used. Preferred gas
compositions are described at U.S. Pat. Nos. 4,393,405, 6,157,162
and 4,977,573 and U.S. patent application Ser. Nos. 09/513,025,
09/447,882, 09/418,052, and 09/588,561, each of which is assigned
to the same assignee and is hereby incorporated by reference into
the present application. The concentration of the fluorine in the
gas mixture may range from 0.003% to 1.00%, and is preferably
around 0.1%. An additional gas additive, such as a rare gas or
otherwise, may be added for increased energy stability, overshoot
control and/or as an attenuator as described in the Ser. No.
09/513,025 application incorporated by reference above.
Specifically, for the F2-laser, an addition of xenon, krypton
and/or argon may be used. The concentration of xenon or argon in
the mixture may range from 0.0001% to 0.1%. For an ArF-laser, an
addition of xenon or krypton may be used also having a
concentration between 0.0001% to 0.1%. For the KrF laser, an
addition of xenon or argon may be used also having a concentration
between 0.0001% to 0.1%.
[0212] Halogen and rare gas injections, total pressure adjustments
and gas replacement procedures are performed using the gas handling
module 106 preferably including a vacuum pump, a valve network and
one or more gas compartments. The gas handling module 106 receives
gas via gas lines connected to gas containers, tanks, canisters
and/or bottles. Some preferred and alternative gas handling and/or
replenishment procedures, other than as specifically described
herein (see below), are described at U.S. Pat. Nos. 4,977,573 and
5,396,514 and U.S. patent application Ser. Nos. 09/447,882,
09/418,052, 09/379,034, 09/734,459, 09/513,025 and 09/588,561, each
of which is assigned to the same assignee as the present
application, and U.S. Pat. Nos. 5,978,406, 6,014,398 and 6,028,880,
all of which are hereby incorporated by reference. A xenon gas
supply may be included either internal or external to the laser
system according to the '025 application, mentioned above.
[0213] Total pressure adjustments in the form of releases of gases
or reduction of the total pressure within the laser tube 102 are
preferably facilitated by using the auxiliary volume 126. A valve
is opened between the auxiliary volume 126 and the gas mixture in
the laser tube 102 when the auxiliary volume 126 is at lower
pressure than the laser tube 102, preferably due to the vacuum pump
being connected to the auxiliary volume before or during the
pressure release. Total pressure adjustments in the form of
increases in the total pressure may be performed using the valves
of the gas handling unit 106 and injecting combinations of gases or
only a single gas such as the buffer gas of helium, neon or a
combination thereof. Total pressure adjustments may be followed by
gas composition adjustments if it is determined that, e.g., other
than the desired partial pressure of halogen gas is within the
laser tube 102 after the total pressure adjustment. Total pressure
adjustments may also be performed after gas replenishment actions,
and may be performed in combination with smaller adjustments of the
driving voltage to the discharge than would be made if no pressure
adjustments were performed in combination.
[0214] The auxiliary volume 126 is connected to the laser tube 102
for releasing gas from the laser tube 102 into the volume 126 based
on control signals received from the processor 111. The processor
111 regulates the release of gases via a valve assembly to the
auxiliary volume 126, and also regulates the delivery of gases or
mixtures of gases to the laser tube 102 via a valve assembly or
system of valves associated with the gas handling unit 106.
[0215] The auxiliary volume preferably includes a reservoir or
compartment having a known volume and preferably having a pressure
gauge attached for measuring the pressure in the auxiliary volume.
Alternatively or in combination with the pressure gauge, a flow
rate controller allows the processor to control the flow rate of
gases from the tube 102 to the auxiliary volume 126, so that the
processor may control and/or determine precisely how much gas is
being released or has been released. The auxiliary volume 126 as
well as the laser tube may also each have means, such as a
thermocouple arrangement, for measuring the temperature of the
gases within the volume 126 and tube 102. The compartment 107 may
be a few to thousands of cm.sup.3 or so in volumetric size
(preferably, the laser tube 102 may be around 42,000 cm.sup.3
volumetrically).
[0216] At least one valve is included for controlling the flow of
gases between the laser tube 102 and the auxiliary volume 126.
Additional valves may be included therebetween. Another valve is
also included between the vacuum pump 128 and the auxiliary volume
126 for controlling access between the vacuum pump 128 and the
auxiliary volume 126. A further valve or valves may be provided
between either or both of the vacuum pump 128 and auxiliary volume
126 and the laser tube 102 and the auxiliary volume 126 for
controlling the atmosphere in the line therebetween, and an
additional pump or the same vacuum pump 128 may be used to evacuate
the line between the laser tube and auxiliary volume either
directly or through the auxiliary volume.
[0217] Predetermined amounts of the gas mixture in the tube 102 are
preferably released into the auxiliary volume 126 from the laser
tube 102 for total pressure releases according algorithms of
preferred embodiments set forth herein. This same auxiliary volume
may be used in partial, mini- or macro-gas replacement operations
such as are set forth above. As an example, the gas handling unit
106 connected to the laser tube 102 either directly or through an
additional valve assembly, such as may include a small compartment
for regulating the amount of gas injected (see above), may include
a gas line for injecting a premix A including 1% F.sub.2:99% Ne,
and another gas line for injecting a premix B including 1% Kr:99%
Ne, for a KrF laser. For an ArF laser, premix B would have Ar
instead of Kr, and for a F.sub.2 laser premix B is not used. Thus,
by injecting premix A and premix B into the tube 102 via the valve
assembly, the fluorine and krypton concentrations (for the KrF
laser, e.g.) in the laser tube 102, respectively, may be
replenished. Then, a certain amount of gas is released
corresponding to the amount that was injected. Additional gas lines
and/or valves may be used for injecting additional gas mixtures.
New fills, partial and mini gas replacements and gas injection
procedures, e.g., enhanced and ordinary micro-halogen injections,
and any and all other gas replenishment actions are initiated and
controlled by the processor 111 which controls valve assemblies of
the gas handling unit 106, laser tube 102, auxiliary volume 126 and
vacuum pump 128 based on various input information in a feedback
loop.
[0218] An exemplary method according to the present invention is
next described for accurately and precisely releasing gas from the
laser tube 102 into the auxiliary volume 126. It is noted that a
similar procedure for accurately and precisely replenishing gases
including injection into the laser tube 102 are preferably used for
injecting small amounts of gases such that significant output beam
parameters are not significantly disturbed, if at all, with each
gas injection. For example, the processor 116, which is monitoring
a parameter indicative of the fluorine concentration in the laser
tube 102, may initiate a micro-halogen injection (.mu.HI) when the
processor 116 determines it is time to increase the halogen
concentration in the gas mixture in the laser tube 102 (further
details of preferred gas replenishment actions are described
above).
[0219] With respect to preferred total pressure releases according
to preferred embodiments herein, the processor 116 determines that
it is time for a pressure release. The processor 116 then sends a
signal that causes a valve to open between the tube 102 and the
volume 126 to gases to flow from the tube 102 to the volume 126
either to a predetermined pressure in the auxiliary volume 126 or
according to a known flow rate and time that the valve is to be
opened. Then, the valve is closed. Preferably, the pressure in the
tube 102 after the release is determined by either a pressure gauge
on the tube 102 or by calculation using the known amount of gas
released and the amount of gas that was in the tube 102 before the
release. A valve between the vacuum pump and the auxiliary volume
is then preferably opened allowing the gas in the volume 126 to be
pumped out of the volume.
[0220] If the pressure in the tube was 3 bar prior to the release
and the tube has 42,000 cm.sup.3, and the release is such that the
pressure in the auxiliary volume was increased to, e.g., around 3
bar after the release, then 0.5.times.[(volume of auxiliary volume
26)/42,000] bar would be the pressure reduction in the tube 102 as
a result of the release. Particular total pressure release or
addition algorithms are set forth below with reference to FIGS.
26-29.
[0221] The above calculation may be performed by the processor 116
to determine more precisely how much gas was released, or prior to
the release, the pressure in the auxiliary volume 126 may be set
according to a calculation by the processor 116 concerning how much
gas should be released based on the information received by the
processor 116 and/or based on pre-programmed criteria. Preferably,
the auxiliary volume is pumped down such that a substantially zero
pressure approximation may be used, or a very low pressure as
measured by a gauge measuring the pressure in the interior of the
volume 126. A correction for difference in temperature between the
gas in the tube 102 and the auxiliary volume 126 may also be
performed by the processor 116 for greater accuracy, or the
temperature within the auxiliary volume may be preset, e.g., to the
temperature within the laser tube 102.
[0222] There may be more than one auxiliary volume like the volume
126, as described above, each having different properties such as
volumetric space. For example, there may be two compartments, one
for gas replacement procedures and one for total pressure releases.
There may be more than two, for still further versatility in the
amounts of gas to be released, and for adjusting driving voltage
ranges corresponding to different gas action algorithms (see
above).
[0223] A general description of the line-narrowing features of the
several embodiments of the present is provided here, followed by a
listing of patent and patent applications being incorporated by
reference as describing variations and features that may used
within the scope of the present invention for providing an output
beam with a high spectral purity or bandwidth (e.g., below 1 pm and
preferably 0.6 pm or less). Exemplary line-narrowing optics
contained in the optics module 110 include a beam expander, an
optional interferometric device such as an etalon and a diffraction
grating, which produces a relatively high degree of dispersion, for
a narrow band laser such as is used with a refractive or
catadioptric optical lithography imaging system. As mentioned
above, the front optics module may include line-narrowing optics as
well (see the U.S. Pat. Nos. 09/715,803, 09/738,849, and 09/718,809
applications, each being assigned to the same assignee and hereby
incorporated by reference).
[0224] For a semi-narrow band laser such as is used with an
all-reflective imaging system, the grating may be replaced with a
highly reflective mirror, and a lower degree of dispersion may be
produced by a dispersive prism. A semi-narrow band laser would
typically have an output beam linewidth in excess of 1 pm and may
be as high as 100 pm in some laser systems, depending on the
characteristic broadband bandwidth of the laser.
[0225] The beam expander of the above exemplary line-narrowing
optics of the optics module 110 preferably includes one or more
prisms. The beam expander may include other beam expanding optics
such as a lens assembly or a converging/diverging lens pair. The
grating or a highly reflective mirror is preferably rotatable so
that the wavelengths reflected into the acceptance angle of the
resonator can be selected or tuned. Alternatively, the grating, or
other optic or optics, or the entire line-narrowing module may be
pressure tuned, such as is set forth in the U.S. Pat. Nos.
09/771,366 and 09/317,527 applications, each of which is assigned
to the same assignee and is hereby incorporated by reference. The
grating may be used both for dispersing the beam for achieving
narrow bandwidths and also preferably for retroreflecting the beam
back toward the laser tube. Alternatively, a highly reflective
mirror is positioned after the grating which receives a reflection
from the grating and reflects the beam back toward the grating in a
Littman configuration, or the grating may be a transmission
grating. One or more dispersive prisms may also be used, and more
than one etalon may be used.
[0226] Depending on the type and extent of line-narrowing and/or
selection and tuning that is desired, and the particular laser that
the line-narrowing optics are to be installed into, there are many
alternative optical configurations that may be used. For this
purpose, those shown in U.S. Pat. Nos. 4,399,540, 4,905,243,
5,226,050, 5,559,816, 5,659,419, 5,663,973, 5,761,236, 6.081,542,
6,061,382, and 5,946,337, and U.S. patent application Ser. Nos.
09/317,695, 09/130,277, 09/244,554, 09/317,527, 09/073,070,
09/452,353, 09/602,184, 09/629,256, 09/599,130, 09/712,367,
09/741,465, 09/774,238, 09/738,849, 09/715,803, 09/718,809,
09/712,367, 09/791,431, each of which is assigned to the same
assignee as the present application, and U.S. Pat. Nos. 5,095,492,
5,684,822, 5,835,520, 5,852,627, 5,856,991, 5,898,725, 5,901,163,
5,917,849, 5,970,082, 5,404,366, 4,975,919, 5,142,543, 5,596,596,
5,802,094, 4,856,018, 5,970,082, 5,978,409, 5,999,318, 5,150,370
and 4,829,536, and German patent DE 298 22 090.3, are each hereby
incorporated by reference into the present application.
[0227] Optics module 112 preferably includes means for outcoupling
the beam 120, such as a partially reflective resonator reflector.
The beam 120 may be otherwise outcoupled such as by an
intra-resonator beam splitter or partially reflecting surface of
another optical element, and the optics module 112 would in this
case include a highly reflective mirror. The optics control module
114 preferably controls the optics modules 110 and 112 such as by
receiving and interpreting signals from the processor 16, and
initiating realignment or reconfiguration procedures (see the '353,
'695, '277, '554, and '527 applications mentioned above).
[0228] The preferred embodiments relate particularly to excimer and
molecular fluorine laser systems configured for adjustment of an
average pulse energy of an output beam of the laser systems by
using gas handling procedures including total pressure adjustments
of the gas mixture in the laser tube 102. To avoid the operation of
laser with a new laser tube and optics at a voltage below an
optimal driving voltage level, or HV.sub.opt, the total gas
pressure after a new fill is advantageously decreased by a total
pressure release, such as has been described above with reference
to FIG. 25, until a desired average pulse energy corresponds to an
input driving voltage within an acceptable range of the high
voltage around HV.sub.opt.
[0229] The halogen and the rare gas concentrations are maintained
constant during laser operation by gas replenishment actions for
replenishing the amount of halogen, rare gas and buffer gas in the
laser tube for KrF and ArF excimer lasers, and halogen and buffer
gas for molecular fluorine lasers, such that these gases are
maintained in a same predetermined ratio as are in the laser tube
102 following a new fill procedure. A resulting decrease in the
total pressure in the tube 102 is achieved by pumping out gases
from the tube 102 through the auxiliary volume 126, as set forth
above. Gas injection actions such as .mu.HIs as understood from the
above, may be advantageously modified into micro gas replacement
procedures, such that the increase in energy of the output laser
beam may be compensated by reducing the total pressure. In
contrast, or alternatively, conventional laser systems would reduce
the input driving voltage so that the energy of the output beam is
at the predetermined desired energy. In this way, the driving
voltage is maintained within a small range around HV.sub.opt, while
the gas procedure operates replenish the gases and maintain the
average pulse energy or energy dose, such as by controlling an
output rate of change of the gas mixture or a rate of gas flow
through the laser tube 102.
[0230] Further stabilization by increasing the average pulse energy
during laser operation may be advantageously performed by
increasing the total pressure of gas mixture in the laser tube up
to P.sub.max. The increase of the total pressure takes place
automatically when gas injections are not compensated by releases
of gas after the injection, or may occur by affirmative further
increases of the total gas pressure in the laser tube 102, which
may be independent of any gas replenishment procedures or
procedures for adjusting the gas composition to a desired
composition, for controlling an increase of the average pulse
energy by total pressure increase. Advantageously, the gas
procedures set forth herein permit the laser system to operate
within a very small range around HV.sub.opt, while still achieving
average pulse energy control and gas replenishment.
[0231] A laser system having a discharge chamber or laser tube 102
with a same gas mixture, total gas pressure, constant distance
between the electrodes and constant rise time of the charge on
laser peaking capacitors of the pulser module 104, also has a
constant breakdown voltage. The operation of the laser has an
optimal driving voltage HV.sub.opt, at which the generation of a
laser beam has a maximum efficiency and discharge stability.
[0232] As mentioned briefly above, an undesirable situation for a
conventional laser tube can occur if the laser is operated at an
input driving voltage much less than HV.sub.opt. Such a low driving
voltage would be outside of the driving voltage range that the
laser of the preferred embodiment is operated within. If a laser is
operated at this low discharge voltage, the discharge is not stable
and the electrodes of the conventional laser system can be eroded
quickly or strongly burned. To avoid the operation of the laser
with a new laser tube and optics at a voltage below HV.sub.opt, the
total gas pressure after a new fill is advantageously reduced in a
laser tube 102 in accord with the preferred embodiment, compared
with a total gas pressure after a new fill of a laser having an
aged laser tube 102 and/or optics. In this way, the desired average
pulse energy is achieved and corresponds to at most only a
deviation from the optimal driving voltage HV.sub.opt. For example,
while a conventional laser is typically reduced to operation at 1.5
kV or the low end of the conventional range and ordinary total
laser tube pressure, the laser of the preferred embodiment is
operated at around 1.7 kV, or around the optimal input driving
voltage, and the laser tube pressure is reduced to a lower
pressure, e.g., around a minimum tolerable pressure P.sub.min.
[0233] A first preferred algorithm according to this procedure is
set forth in the flow diagram shown at FIG. 26. Referring to FIG.
26, at step S31 parameters are set including E.sub.S, U.sub.S,
.DELTA.E.sub.S, .DELTA.P, and P.sub.min, wherein: [0234] E.sub.S
corresponds to a desired average output pulse energy of laser
oscillation, which is preferably around 10 mJ; [0235] U.sub.S
corresponds to a desired driving voltage for laser operation with
average pulse energy E.sub.S, which is preferably at or near
HV.sub.opt, and as mentioned, HV.sub.opt, and correspondingly
U.sub.S, is preferably around 1.7 kV; [0236] .DELTA.E.sub.S
corresponds to a desired tolerance of average pulse energy by laser
operation at driving voltage U.sub.S, and is preferably around 0.5
mJ or less; [0237] .DELTA.P corresponds to a predefined amount of
released gas mixture, e.g., around 100 mbar may be released in
steps; and [0238] P.sub.min corresponds to a predefined minimal
allowed value of the total gas pressure in the laser tube 102, and
may be between 2200 and 2800 mbar, and the minimum pressure
P.sub.min will depend on the gas mixture of the laser system, as so
may vary.
[0239] At step S32, a new fill is performed, such as has been
previously described above and/or is understood by those skilled in
the art. At step S33, the laser is operated at a constant or
substantially constant driving voltage of U.sub.S and a 100% duty
cycle. The output average pulse energy E is then measured using an
energy detector at step S34, wherein the measured energy is then
preferably sent to the processor 116 of FIG. 25.
[0240] The processor 116 then determines whether the measured
energy E is above the tolerance range, i.e., whether
E>E.sub.S+.DELTA.E.sub.S. If at step S35, the processor 116
determines that the energy E is above the tolerance limit
E.sub.S+.DELTA.E.sub.S, then at step S36, a release of pressure
from the laser tube 102 in an amount .DELTA.P is performed. If the
pressure P in the tube 102 after the pressure release is then
determined at step S37 to be greater than P.sub.min, then the
procedure returns to step S33. If the pressure P in the tube 102
after the pressure release is, however, then determined at step S38
to be less than or equal to P.sub.min, then the gas optimization
procedure is ended at step SF. If after the energy measurement at
step S34, the processor 116 determines that the measured energy E
is within or below the tolerance range, i.e.,
E<E.sub.S+.DELTA.E.sub.S, at step S39, then the gas optimization
procedure is ended at step SF.
[0241] A second preferred algorithm according to the gas
optimization procedure of the preferred embodiment is set forth in
the flow diagram shown at FIG. 27. Referring to FIG. 27, at step
S41 parameters are set including E.sub.S, U.sub.S, .DELTA.E.sub.S,
Q, and P.sub.min, wherein: [0242] E.sub.S corresponds to a desired
average output pulse energy of laser oscillation; [0243] U.sub.S
corresponds to a desired driving voltage for laser operation with
average pulse energy E.sub.S, which is preferably at or near
HV.sub.opt; [0244] .DELTA.E.sub.S corresponds to a desired
tolerance of average pulse energy by laser operation at driving
voltage U.sub.S; [0245] Q corresponds to a factor determined based
on a relationship between a desired decreasing of the average laser
pulse energy and a demanded value of released pressure of the gas
mixture, and Q may ordinarily be between 2 and 4; and [0246]
P.sub.min corresponds to a predefined minimal allowed value of the
total gas pressure in the laser tube 102, while preferred values of
some of these quantities has been mentioned above.
[0247] At step S42, a new fill is performed, such as has been
previously described above and/or is understood by those skilled in
the art. At step S43, the laser is operated at a constant or
substantially constant driving voltage of U.sub.S and a 100% duty
cycle. The output average pulse energy E is then measured using an
energy detector at step S44, wherein the measured energy is then
preferably sent to the processor 116 of FIG. 25.
[0248] The processor 116 then determines whether the measured
energy E is above the tolerance range, i.e., whether
E>E.sub.S+.DELTA.E.sub.S. If at step S45, the processor 116
determines that the energy E is above the tolerance limit
E.sub.S+.DELTA.E.sub.S, then at step S46, an amount of pressure
.DELTA.P to be released is calculated by the processor 116. At step
S46, an energy differential .delta.E is calculated as
.delta.E=(E-E.sub.S)/E. Then, a pressure differential .delta.P is
calculated as .delta.P=.delta.E/Q. Then, an amount of pressure
release .DELTA.P is calculated as
.DELTA.P=P.times..delta.P=P.times.(E-E.sub.S)/(Q.times.E), where P
is the pressure in the tube prior to the pressure release
.DELTA.P.
[0249] Then at step S47, a release of pressure from the laser tube
102 in the calculated amount .DELTA.P is performed. If the pressure
P in the tube 102 after the pressure release is then determined at
step S48 to be greater than P.sub.min, then the procedure returns
to step S43. If the pressure P in the tube 102 after the pressure
release is, however, then determined at step S49 to be less than or
equal to P.sub.min, then the gas optimization procedure is ended at
step SF. If after the energy measurement at step S44, the processor
116 determines that the measured energy E is within or below the
tolerance range, i.e., E.ltoreq.E.sub.S+.DELTA.E.sub.S, at step
S50, then the gas optimization procedure is ended at step SF.
[0250] FIG. 27 shows an alternative algorithm to that shown at FIG.
26, wherein the adjustment of the average pulse energy by means of
decreasing of the total gas pressure in the laser tube is performed
by a pressure release in an amount which is calculated at step S46
prior to the release at step S47. In contrast, in the algorithm
shown at FIG. 26, the pressure release amount .DELTA.P is
determined in advance.
[0251] FIG. 28 shows several plots of average laser output energy
versus pressure release steps of same amounts of release pressure
for various values of the Q factor referred to above and used in
the determination of the amount of pressure to be released at step
S46 of the algorithm of FIG. 27. The graph at FIG. 28 shows that
for a laser operating with a Q factor of Q=2, the average output
pulse energy is reduced by about 2 mJ, while for a same pressure
release amount for a laser operating at Q factor Q=9, the average
output pulse energy is reduced by only about 0.4 mJ. The reduction
in average output pulse energy varies in between 2 mJ and 0.4 mJ
for a same pressure release amount for lasers operating at Q
factors between Q=3 and Q=8. FIG. 28 thus clearly shows that the
average pulse energy reduces for a same pressure release amount by
a different amount depending on the Q factor of the laser system.
In order then to reduce the output energies for lasers operating at
different Q values by a same energy amount, the amount of the
pressure release is advantageously varied according the algorithm
of FIG. 27, as shown in the calculation step S46, wherein the
calculated amount is dependent on the q factor. The accuracy of the
approach to reducing the average pulse energy to the desired pulse
energy and number of steps of such procedures is thus improved
according to the algorithm of FIG. 27.
[0252] A third preferred algorithm according to the gas
optimization procedure of the preferred embodiment is set forth in
the flow diagram shown at FIG. 29. Referring to FIG. 29, at step
S61 parameters are set including E.sub.S, U.sub.S, .DELTA.E.sub.S,
Q, P.sub.min and P.sub.max, wherein: [0253] E.sub.s corresponds to
a desired average output pulse energy of laser oscillation; [0254]
U.sub.S corresponds to a desired driving voltage for laser
operation with average pulse energy E.sub.S, which is preferably at
or near HV.sub.opt; [0255] .DELTA.E.sub.S corresponds to a desired
tolerance of average pulse energy by laser operation at driving
voltage U.sub.S; [0256] Q corresponds to a factor determined based
on a relationship between a desired decreasing of the average laser
pulse energy and a demanded value of released pressure of the gas
mixture; [0257] P.sub.min corresponds to a predefined minimum
allowed value of the total gas pressure in the laser tube 102; and
[0258] P.sub.max corresponds to a predefined maximum allowed value
of the total gas pressure in the laser tube 102, and P.sub.max
would typically be greater than 3 bar.
[0259] At step S62, a new fill is performed, such as has been
previously described above and/or is understood by those skilled in
the art. At step S63, the laser is operated at a constant or
substantially constant driving voltage of U.sub.S and a 100% duty
cycle. The output average pulse energy E is then measured using an
energy detector at step S64, wherein the measured energy is then
preferably sent to the processor 116 of FIG. 25.
[0260] The processor 116 then determines whether the measured
energy E is above the tolerance range, i.e., whether
E>E.sub.S+.DELTA.E.sub.S. If at step S65, the processor 116
determines that the energy E is above the tolerance limit
E.sub.S+.DELTA.E.sub.S, then at step S66, an amount of pressure
.DELTA.P to be released is calculated by the processor 116. At step
S66, an energy differential .delta.E is calculated as
.delta.E=(E-E.sub.S)/E. Then, a pressure differential .delta.P is
calculated as .delta.P=.delta.E/Q. Then, an amount of pressure
release .DELTA.P is calculated as
.DELTA.P=P.times..delta.P=P.times.(E-E.sub.S)/(Q.times.E), where P
is the pressure in the tube prior to the pressure release
.DELTA.P.
[0261] Then at step S67, a release of pressure from the laser tube
102 in the calculated amount .DELTA.P is performed. If the pressure
P in the tube 102 after the pressure release is then determined at
step S68 to be greater than P.sub.min, then the procedure returns
to step S64. If the pressure P in the tube 102 after the pressure
release is, however, then determined at step S69 to be less than or
equal to P.sub.min, then the gas optimization procedure is ended at
step SF.
[0262] If after the energy measurement at step S64, the processor
116 determines that the measured energy E is within or below the
tolerance range, i.e., E.ltoreq.E.sub.S+.DELTA.E.sub.S, at step
S70, an amount of pressure .DELTA.P to be added into the laser tube
102 is calculated by the processor 116. At step S71, an energy
differential .delta.E is calculated as .delta.E=(E-E.sub.S)/E.
Then, a pressure differential .DELTA.P is calculated as
.DELTA.P=.delta.E/Q. Then, an amount of pressure addition .DELTA.P
is calculated as
.DELTA.P=P.times..delta.P=P.times.(E-E.sub.s)/(Q.times.E), where P
is the pressure in the tube prior to the pressure add .DELTA.P.
[0263] Then at step S72, an addition of pressure into the laser
tube 102 in the calculated amount .DELTA.P is performed. If the
pressure P in the tube 102 after the pressure addition is then
determined at step S73 to be less than P.sub.max, then the
procedure returns to step S64. If the pressure P in the tube 102
after the pressure addition is, however, then determined at step
S74 to be greater than or equal to P.sub.max, then the gas
optimization procedure is ended at step SF.
[0264] FIG. 29 shows an alternative algorithm to those shown at
FIGS. 26 and 27, wherein the adjustment of the average pulse energy
by means of decreasing and increasing the total gas pressure in the
laser tube 102 is performed by a pressure release or pressure
addition, respectively, in an amount which is calculated at step
S66 or step S71 prior to the release or addition at step S47 or
step S72. In contrast, in the algorithm shown at FIG. 26, the
pressure release amount .DELTA.P is determined in advance, and
there are no pressure addition steps in either of the algorithms of
FIG. 26 or 27.
[0265] The algorithm of FIG. 29 may be modified to an algorithm for
incorporating total pressure adjustments within an average pulse
energy control algorithm other than just after a new fill.
Conventional average pulse energy control algorithms are known by
those skilled in the art and steps involved in those procedures
that are preferably combined with those of the following algorithm
and those steps involved in the algorithms shown at FIGS. 26-27 and
29 are not shown or described here. The Ser. Nos. 09/588,561 and
09/734,459 applications, incorporated herein by reference above,
describe energy control algorithms including burst overshoot
control (see particularly the '561 application) and average pulse
energy control components of a preferred algorithm that may be
combined with the algorithm described here which includes pressure
adjustments for controlling the average pulse energy.
[0266] Parameters have already been set according to the algorithms
set forth above, and may be adjusted for this present algorithm,
including E.sub.S, U.sub.S, .DELTA.E.sub.S, Q, P.sub.min and
P.sub.max, wherein: [0267] E.sub.S corresponds to a desired average
output pulse energy of laser oscillation; [0268] U.sub.S
corresponds to a desired driving voltage for laser operation with
average pulse energy E.sub.S, which is preferably at or near
HV.sub.opt; [0269] .DELTA.E.sub.S corresponds to a desired
tolerance of average pulse energy by laser operation at driving
voltage U.sub.S; [0270] Q corresponds to a factor determined based
on a relationship between a desired decreasing of the average laser
pulse energy and a demanded value of released pressure of the gas
mixture; [0271] P.sub.min corresponds to a predefined minimum
allowed value of the total gas pressure in the laser tube 2; and
[0272] P.sub.max corresponds to a predefined maximum allowed value
of the total gas pressure in the laser tube 2.
[0273] At step S163, the laser is operated at a constant or
substantially constant driving voltage of U.sub.S and a 100% duty
cycle. The output average pulse energy E is then measured using an
energy detector at step S164, wherein the measured energy is then
preferably sent to the processor 16 of FIG. 25. The processor 16
may have and preferably does have other procedures available to it
for controlling the average pulse energy upon measurement at step
S164, such as gas replenishment, but in the present algorithm, the
energy may be adjusted upon initiation by the processor 16 of a
pressure adjustment.
[0274] The processor 16 determines whether the measured energy E is
above the tolerance range, i.e., whether
E>E.sub.S+.DELTA.E.sub.S or whether the measured energy E is
below the tolerance range, i.e., whether
E<E.sub.S-.DELTA.E.sub.S. If at step S165, the processor 16
determines that the energy E is above the tolerance limit
E.sub.S+.DELTA.E.sub.S, then at step S1166, an amount of pressure
.DELTA.P to be released is calculated by the processor 16. At step
S166, an energy differential .delta.E is calculated as
.delta.E=(E-E.sub.S)/E. Then, a pressure differential .delta.P is
calculated as .delta.P=.delta.E/Q. Then, an amount of pressure
release .DELTA.P is calculated as
.DELTA.P=P.times..delta.P=P.times.(E-E.sub.S)/(Q.times.E), where P
is the pressure in the tube prior to the pressure release .DELTA.P.
Alternatively, a pressure amount .DELTA.P may be determined in
advance, such as is described above with reference to the algorithm
shown at FIG. 26.
[0275] Then at step S167, a release of pressure from the laser tube
2 in the calculated amount .DELTA.P is performed. If the pressure P
in the tube 2 after the pressure release is then determined at step
S168 to be greater than P.sub.min, then the procedure returns to
step S164. If the pressure P in the tube 2 after the pressure
release is, however, then determined at step S169 to be less than
or equal to P.sub.min, then the pressure release option is disabled
temporarily following step 169, at least until the pressure in the
tube 2 is later increased to above P.sub.min. Such an increase of
the total pressure P may, e.g., occur as gas injections are
performed without corresponding pressure releases being performed
or as affirmative pressure additions coupled with other gas actions
or according to the operations set forth at steps S170-S172.
[0276] If after the energy measurement at step S164, the processor
16 determines that the measured energy E is below the tolerance
range, i.e., E<E.sub.S-.DELTA.E.sub.S, at step S170, an amount
of pressure .DELTA.P to be added into the laser tube 2 is
calculated by the processor 16. At step S171, an energy
differential .delta.E is calculated as .delta.E=(E-E.sub.S)/E.
Then, a pressure differential .delta.P is calculated as
.delta.P=.delta.E/Q. Then, an amount of pressure addition .DELTA.P
is calculated as
.DELTA.P=P.times..delta.P=P.times.(E-E.sub.S)/(Q.times.E), where P
is the pressure in the tube prior to the pressure add .DELTA.P.
Alternatively, a pressure amount .DELTA.P may be determined in
advance, such as is described above with reference to the algorithm
shown at FIG. 26.
[0277] Then at step S172, an addition of pressure into the laser
tube 2 in the calculated amount .DELTA.P is performed. If the
pressure P in the tube 2 after the pressure addition is then
determined at step S173 to be less than P.sub.max, then the
procedure returns to step S164. If the pressure P in the tube 2
after the pressure addition is, however, then determined at step
S174 to be greater than or equal to P.sub.max, then the pressure
addition option is disabled temporarily following step 174, at
least until the pressure in the tube 2 is later decreased to below
P.sub.max. Such a decrease of the total pressure P may, e.g., occur
or as affirmative pressure releases coupled with other gas actions
or according to the operations set forth at steps S165-S167.
[0278] It is here noted that modifications of the methods described
above may be made. For example, the energy of the laser beam may be
continuously maintained within a tolerance range around the desired
energy by adjusting the input driving voltage. The input driving
voltage may then be monitored. When the input driving voltage is
above or below the optimal driving voltage HV.sub.opt by a
predetermined or calculated amount, then a total pressure addition
or release, respectively, may be performed to adjust the input
driving voltage a desired amount, such as closer to HV.sub.opt, or
otherwise within a tolerance range of the input driving voltage.
The total pressure addition or release may be of a predetermined
amount of a calculated amount, such as described above. In this
case, the desired change in input driving voltage may be determined
to correspond to a change in energy which would then be compensated
by the calculated or predetermined amount of gas addition or
release, such that similar calculation formulas may be used as
those set forth above.
[0279] By incorporating pressure adjustments into an average pulse
energy control algorithm according to a preferred embodiment, the
range of driving voltages applied to the discharge may be
advantageously reduced, such that the driving voltages applied to
the discharge do not vary much from the optimal driving voltage
HV.sub.opt, even as a result of the aging of the gas mixture, and
without reducing a duration between new fills. Another advantage
that may be realized by itself or in combination with reducing the
driving voltage variation range, is that a duration between new
fills may be extended by utilizing pressure additions for
increasing output energy (over operating at a lower pressure) in
combination with gas replenishment techniques such as halogen
injections and gas replacements as set forth below. This latter
advantageous feature is described below according to a preferred
embodiment.
[0280] Several driving voltage levels (HV.sub.i) can be defined
wherein particular gas actions are predetermined to be performed.
The processor 16 monitors the driving voltage and causes the gas
supply unit to perform gas injections of varying degrees, partial
and mini gas replacements of varying degrees, and total pressure
adjustments of various degrees or in constant amounts, depending on
the value of the driving voltage, or which preset range the current
operating driving voltage is in (left hand side y-axis of FIG. 23
of the Ser. No. 09/734,459 application), based on such parameters
as time, pulse count and/or total input electrical energy to the
discharge, etc. (see the '561 application, mentioned above) x-axis
of FIG. 23 of the Ser. No. 09/734,459 application).
[0281] An example in accord with the present invention is next
described. The laser system is configured to be capable of
operating at driving voltages between HV.sub.min and HV.sub.max.
The actual operating minimum and maximum driving voltages are set
to be in a much smaller range between HV.sub.1 and HV.sub.7, as
illustrated by the broken ordinate axis. An advantage of this
preferred embodiment is that the range HV.sub.1 to HV.sub.7 itself
may be reduced to a very small window such that the operating
voltage is never varied greatly during operation of the laser.
Where this operating range itself lies between HV.sub.min and
HV.sub.max, i.e., the actual voltage range (in Volts) corresponding
to the range may be adjusted, e.g., to increase the lifetimes of
the optical components of the resonator and the laser tube, e.g.,
such as by adjusting an output energy attenuating gas additive (see
the '025 application, mentioned above) or using an extra-resonator
attenuator (see U.S. patent application No. 60/178,620, which is
assigned the same assignee and is hereby incorporated by
reference). Also advantageously, the operating driving voltage
range between HV.sub.1 and HV.sub.7 may be reduced so that the
driving voltage varies less from HV.sub.opt than previous systems,
while maintaining or extending the lifetime of the laser tube,
optics and/or gas mixture between new fills.
[0282] The right hand side ordinate axis of FIG. 23 of the Ser. No.
09/734,459 application may be modified to show total pressures
within the laser tube 2 during a lifetime of the gas mixture
(between new fills), and corresponding with driving voltage levels
shown on the left hand side ordinate axis. At the lowest driving
voltage used in this system, i.e., HV.sub.1, the pressure is
preferably as low as P.sub.min. By having a lowest tolerable
pressure in the tube when, e.g., a new fill has been recently
performed, the driving voltage level HV.sub.1 can be higher (and
closer to HV.sub.opt) than for a system operating with a constant
total pressure, while providing a same desired average pulse
energy.
[0283] At the highest driving voltage used in this system, i.e.,
HV.sub.7, the pressure in the tube 2 is preferably as high as
P.sub.max. By having a highest tolerable pressure in the tube when,
e.g., the gas mixture is reaching the end of its lifetime before a
new fill will be performed, the driving voltage level HV.sub.7 can
be lower (and closer to HV.sub.opt) than for a system operating
with a constant total pressure, while providing a same desired
average pulse energy. Alternatively, the driving voltage range can
be extended to provide a longer gas lifetime. Preferably, the total
pressure is incremented along with the driving voltage range used
as the gas mixture ages, to extend the durations of use of each
driving voltage range.
[0284] The coordinate axis of FIG. 23 of the Ser. No. 09/734,459
application denotes gas actions that may be performed, based on one
or more accumulated parameters, when the driving voltage is in each
interval. In addition, as is evident from the increasing of the
laser tube total pressure shown on the right hand side ordinate
axis, pressure additions are also preferably performed as the gas
mixture ages, or as the system progresses from left to right on the
coordinate axis. The general order of performance of the gas
actions is from left to right as the gas mixture ages. However,
when each gas action is performed, the driving voltage is checked,
and the next gas action that may be performed may correspond to the
same driving voltage range, or a different one denoted to the left
or the right of that range. For example, after a PGR is performed
(when it is determined that the driving voltage is above HV.sub.5),
the driving voltage may be reduced to between HV.sub.2 and
HV.sub.3, and so the system would return to ordinary .mu.HI and
MGR.sub.1 gas control operations. Also, upon performance of a
pressure increase, the driving voltage range may be reduced from a
higher voltage to a lower voltage range, depending on the state of
the system after the pressure increase.
[0285] Within the operating range between HV.sub.1 and HV.sub.7,
several other ranges are defined. For example, when the driving
voltage HV is between HV.sub.1 and HV.sub.2 (i.e.,
HV.sub.1<HV<HV.sub.2), and the total tube pressure is between
P.sub.min and P.sub.1, no gas actions are performed as there is a
sufficient amount of halogen in the gas mixture. When the driving
voltage is between HV.sub.2 and HV.sub.3 (i.e.,
HV.sub.2<HV<HV.sub.3), and the total tube pressure is between
P.sub.1 and P.sub.2, MGR.sub.1 and ordinary .mu.HI are performed
periodically based on the accumulated parameter(s) (i.e., input
electrical energy to the discharge, time, and/or pulse count,
etc.). This is the ordinary range of operation of the system in
accord with the preferred embodiment.
[0286] When the driving voltage is between HV.sub.3 and HV.sub.4
(i.e., HV.sub.3>HV>HV.sub.4), and the total tube pressure is
between P.sub.2 and P.sub.3, one or both of the injection amounts
of the .mu.HIs and the MGRs with corresponding gas releases is
increased. In this example, only the .mu.HIs are increased. Thus,
the range between HV.sub.3 and HV.sub.4 is the range within which
enhanced .mu.HIs are performed, which are preferably halogen
injections of greater amount or reduced duration than ordinary
.mu.HIs, and the same MGR amounts as in the previous range between
HV.sub.2 and HV.sub.3 are maintained.
[0287] To be clear, enhanced .mu.HIs may differ from ordinary
.mu.HIs in one or both of two ways. First, the amount per injection
may be increased. Second, the interval between successive .mu.HIs
may be decreased.
[0288] The range between HV.sub.4 and HV.sub.5 (i.e.,
HV.sub.4<HV<HV.sub.5), and between total tube pressures
P.sub.3 and P.sub.4, represents a new range within which one or
both of the injection amounts of the .mu.HIs and the MGRs with
corresponding gas releases is increased (or the duration between
successive actions is reduced). In this example, only the MGRs are
increased as compared with the range HV.sub.3 to HV.sub.4. Thus, an
enhanced amount of halogen gas is injected (with corresponding
release of gases) during each MGR.sub.2 than the ordinary amount
MGR.sub.1 when the driving voltage is in the range between HV.sub.4
and Hv.sub.5. Alternatively, or in combination with replacing the
gas in larger amounts, the mini gas replacements MGR.sub.2 are
performed at shorter intervals than the MGR.sub.1 are performed. In
each of the preferred and alternative MGR.sub.2 procedures, the
contaminants in the discharge chamber are reduced at smaller
intervals (e.g., of accumulated input energy to the discharge,
pulse count and/or time, among others) compared with the MGR.sub.1
procedures that are performed at the lower driving voltage range
between HV.sub.3 and HV.sub.4. The .mu.HIs are also preferably
performed periodically in this range to recondition the gas
mixture. It is noted here that several ranges wherein either or
both of the amounts injected during the .mu.HIs and MGRs is
adjusted may be defined each corresponding to a defined driving
voltage range. Also, as mentioned above with respect to monitoring
the pressure (and optionally the temperature) in the accumulator
(and optionally the laser tube), the amount injected may be
adjusted for each injection.
[0289] When the driving voltage is above HV.sub.5 (i.e.,
HV.sub.5<HV<HV.sub.6), a still greater gas replacement PGR is
implemented. PGR may be used to replace up to ten percent or more
of the gas mixture. Certain safeguards may be used here to prevent
unwanted gas actions from occurring when, for example, the laser is
being tuned. One is to allow a certain time to pass (such as
several minutes) after the HV.sub.5 level is crossed before the gas
action is allowed to be performed, thus ensuring that the driving
voltage actually increased due to gas mixture degradation. When the
driving voltage goes above HV.sub.6, then it is time for a new fill
of the laser tube. It is noted here that the magnitudes of the
driving voltages ranges shown in FIG. 23 of the Ser. No. 09/734,459
are not necessarily are not necessarily drawn to scale.
[0290] A flow sequence for performing ordinary and enhanced
.mu.HIs, MGRs and PGRs in accord with the preferred embodiment and
the example just set forth above will now be described, wherein
FIG. 24 of the Ser. No. 09/734,059 application illustrates this
flow sequence. The procedure starts with a new fill, wherein the
discharge chamber is filled with an optimal gas mixture. The laser
can thereafter be in operation for industrial applications, in
stand-by mode or shut off completely. A driving voltage check
(HV-check) is performed after the current driving voltage (HV) is
measured.
[0291] The measured driving voltage (HV) is compared with
predetermined values for HV.sub.1 through HV.sub.7. The processor
determines whether HV lies between HV.sub.1 and HV.sub.2 (i.e.,
HV.sub.1<HV<HV.sub.2) and thus path (1) is followed and no
gas actions are to be performed and the procedure returns to the
previous step. Although not shown, if the HV lies below HV.sub.1,
then a procedure may be followed to decrease the halogen
concentration in the laser tube, such as by releasing some laser
gas and/or injecting some buffer gas from/into the laser tube.
Alternatively, if the total pressure is not at or below P.sub.min,
then a pressure release may be performed according to the algorithm
set forth at either of FIGS. 26-27, described above.
[0292] If the processor determines that the HV lies between
HV.sub.2 and HV.sub.3, then the system is within the ordinary
operating driving voltage band. If it is within the ordinary
operating band, then path (2) is followed whereby ordinary .mu.HIs
and MGR.sub.1 may be performed based preferably on time, input
electrical energy to the discharge and/or pulse count intervals as
predetermined by the expert system based on operating conditions.
Again each gas action may be adjusted depending on the calculated
partial pressure or number of halogen molecules in the laser tube,
as described above.
[0293] The .mu.HIs and MGR.sub.1 performed when path (2) is
followed may be determined in accordance with any method set forth
in U.S. patent application Ser. No. 09/588,561, already
incorporated by reference. If HV is not within the ordinary
operating band, then it is determined whether HV lies below
HV.sub.2 (i.e., HV<HV.sub.2). If HV is below HV.sub.2, then path
(2) is followed and no gas actions are performed.
[0294] If HV lies between HV.sub.3 and HV.sub.4 (i.e.,
HV.sub.3<HV<HV.sub.4), then path (3) is followed and enhanced
.mu.HI and MGR.sub.1 may be performed again based on the value or
values of the time, pulse count and/or applied electrical energy to
the discharge counter(s) being used. The precise amounts and
compositions of gases that are injected and those that are released
are preferably determined by the expert system and will depend on
operating conditions.
[0295] If HV lies between HV.sub.4 and HV.sub.5 (i.e.,
HV.sub.4<HV<HV.sub.5), then path (4) is followed and enhanced
.mu.HI and MGR.sub.2 may be performed depending on checking the
values of the counters. Again, the precise amounts and compositions
of gases that are injected and those that are released are
preferably determined by the expert system and will depend on
operating conditions.
[0296] If HV lies between HV.sub.5 and HV.sub.6 (i.e.,
HV.sub.5<HV<HV.sub.6), then PGR is performed. If HV lies
above HV6 (i.e., HV.sub.6<HV), then a new fill is performed.
[0297] After any of paths (2)-(5) is followed and the corresponding
gas actions are performed, and preferably after a specific settling
time, the method returns to the step of determining the operating
mode of the laser and measuring and comparing HV again with the
predetermined HV levels HV.sub.1 through HV.sub.7.
[0298] The setting of all of these different driving voltage and
laser tube total pressure levels and time, applied electrical
energy to the discharge and/or pulse count schedules can be done
individually or can refer to the computer controlled data base
where they are stored for different operation conditions. The
operation of the laser at different HV-levels under different
operation conditions such as continuous pulsing or burst mode may
be taken into consideration.
[0299] Also in accord with the preferred embodiment, a partial new
fill procedure may be performed. As shown in FIG. 23 of the Ser.
No. 09/734,459 application, an additional HV range is established
which lies above the PGR range 5 and yet below the driving voltage
threshold value HV.sub.7. When the processor determines that the
high voltage is above HV.sub.6, then either a new fill or a partial
new fill will be performed depending on whether the high voltage is
at or below HV.sub.7 wherein a partial new fill is to be performed,
or is above HV.sub.7, wherein a total new fill is performed.
Alternatively, if the total pressure is still below P.sub.max, a
pressure addition may be performed to extend laser operation until
the total pressure reaches P.sub.max, when total pressure additions
are disabled following step S174 according to the above
description.
[0300] When a total new fill is performed, substantially 100% of
the gas mixture is emptied from the discharge chamber and a totally
fresh gas mixture is introduced into the laser chamber. However,
when a partial new fill is performed, only a fraction (5% to 70% or
around 0.15 bar to 2 bar, as examples) of the total gas mixture is
released. More particularly preferred amounts would be between 20%
and 50% or 0.6 bar to 1.5 bar. A specifically preferred amount
would be around 1 bar or around 30% of the gas mixture. Experiments
have shown that implementing a partial new fill procedure wherein 1
bar is exchanged increases the gas lifetime by as much as five
times over not having the procedure.
[0301] The amount that may be released is an amount up to which a
substantial duration of time is used to get the gas out with a
pump, and so the amount may be more than 50%, and yet may take
substantially less time than a total new fill. Thus, a partial new
fill procedure has the advantage that a large amount of aged gas is
exchanged with fresh gas in a short amount of time, thus increasing
wafer throughput when the laser is being used in lithographic
applications, for example.
[0302] When the processor determines that the high voltage is above
HV.sub.6, a determination is made whether the high voltage is at or
below HV.sub.7. If the answer is yes, i.e., that the high voltage
is at or below HV.sub.7, then a partial new fill is initiated,
whereby less than substantially 100% of the gas mixture is taken
out of the discharge chamber and replaced with fresh gas.
Advantageously, the system is only taken offline for a short time
compared with performing a total new fill. If the answer is no,
i.e., that the high voltage is above HV.sub.7, then a new fill is
performed. As mentioned, experiments have shown that the gas
lifetime can be improved by as much as five times before the new
fill range would be reached when the partial new fill procedure is
implemented.
[0303] It is to be understood that a system not using all of the
ranges 1-6 and the new fill/partial new fill procedures of range 6
may be advantageously implemented. For example, a system that uses
only a single one of the ranges with the partial new fill and new
fill may be used, and the gas lifetime improved. With some ranges
removed, the partial new fill range may be moved to a lower
threshold high voltage. In addition, fewer or more than the total
pressure ranges shown may be used, and the pressure ranges may be
used more than once over the driving voltage range from HV.sub.1 to
HV.sub.7. For example, the entire total pressure range may be used
during one range or fewer than all of the driving voltage ranges,
and then the total pressure range from P.sub.min to P.sub.max may
be used again at a higher driving voltage range, and so on. It is
preferred that all of the ranges and corresponding gas actions be
used for optimum laser system performance.
[0304] The combination of all of these different kinds of gas
control and replenishment mechanisms involves harmonizing many
factors and variables. Combined with the expert system and
database, the processor controlled laser system of the preferred
embodiment offers an extended gas lifetime before a new fill is
necessary. In principle, bringing down the laser system for new
fill might be totally prevented. The lifetime of the laser system
would then depend on scheduled maintenance intervals determined by
other laser components such as those for laser tube window or other
optical components exchange. Again, as mentioned above, even the
lifetimes of the laser tube and resonator components may be
increased to increase the intervals between downtime periods.
[0305] During the laser operation, it is important to keep the
ratio of the concentrations of the gas components of the gas
mixture constant at their desired ratios as set forth above by
example for the ArF, KrF and F.sub.2 laser systems. The halogen
injections preferably compensate not only the depleting of F.sub.2
during the laser operation, but also the static degradation of the
gas mixture in the laser tube 2, and in any case the degradation of
the optical components, too.
[0306] If the fluorine level is not monitored and maintained at the
desired level, e.g., around 0.1%, then by compensating the aging of
the optics and laser tube and contaminant build-up in the laser
tube 2, the gas mixture can have a larger amount of fluorine than
the desired amount, while the average energy is at the desired
level. If this is done, then various laser oscillation parameters
can vary from their desired values during the laser operation as a
result of the increased fluorine concentration, and the gas
lifetime and lifetime of the laser tube can be reduced. It is
therefore desired to keep the fluorine concentration constant at
the desired value, and to vary the total pressure and driving
voltage within its limited range according to the preferred
embodiments. The halogen and the rare gas concentrations,
particularly, for the ArF and KrF lasers, and the halogen
concentration for the F.sub.2 laser, are preferably maintained
constant during the laser operation, wherein the replenished
amounts of halogen and rare gas, and preferably buffer gas, in the
laser tube 2 are maintained in the same ratio as just after a new
fill procedure.
[0307] When pressure releases are performed, the reduction of the
total gas pressure in the laser tube 2 is preferably achieved by
pumping through the auxiliary volume 26 (see FIG. 25). This
auxiliary volume 26 is preferably used because the amount of the
released gas mixture is very small, and thus a more precise
determination of the release amount is desired, compared, e.g., to
pumping the laser tube 2 during a new fill or partial new fill.
[0308] Many variations are possible according to the preferred
embodiments and many alternative embodiments can be understood by
those skilled in the art. Micro halogen injections, as described
above, may be advantageously replaced with micro replacements or
constant replacements of gas mixture, i.e., a pressure release is
performed is association with the halogen injection. In this case,
the gas procedure may involve working with a single parameter,
i.e., a rate at which the gas mixture is changed or a rate of gas
flow through the laser tube 2.
[0309] This gas flow rate can have a constant value. This value can
be preferably as large as possible to extend the lifetime of the
gas mixture. In addition, the gas flow rate can be made to depend
on the time, pulse count, input energy to the discharge, etc., such
that the rate may be increased according to a progression of one or
more of these parameters. The gas flow rate may also depend on the
duty cycle of the laser, such that, e.g., the rate is increased at
higher duty cycles.
[0310] In addition, total pressure increases during the laser
operation may be performed according to the gas flow rate, such
that when the gas flow rate of micro gas replacements exceeds a
threshold value, then the pressure is increased. The increasing of
total gas pressure in the laser tube 2 may also be made to depend
on the high voltage of the laser operation. The total pressure
increases can begin at the start of laser operation, e.g., after a
new fill, or can be started some time, pulse count or input energy
to the discharge amount after the new fill. The time, pulse count
or total input energy to the discharge amount when the pressure
increases are started may also depend on the duty cycle.
Preferably, the total gas pressure increases themselves increase at
a constant value from when they are started, and may depend again
on the time, pulse count or total input energy to the discharge
from the start of laser operation or the start of the pressure
increases, or on the duty cycle or on the driving voltage level or
range of operation of the laser system.
[0311] Methods and apparatuses are provided below, such as a narrow
band excimer or molecular fluorine laser system including an
oscillator and an amplifier, wherein the oscillator produces a
sub-250 nm beam having a linewidth less than 1 pm and the amplifier
increases the power of the beam above a predetermined amount, such
as more than one or several Watts. In general, the molecular
fluorine laser will be used as an example. The oscillator includes
a discharge chamber filled with a laser gas including molecular
fluorine and a buffer gas, electrodes within the discharge chamber
connected to a discharge circuit for energizing the molecular
fluorine, and a resonator including the discharge chamber and
line-narrowing optics for generating the laser beam having a
wavelength around 248 nm, 193 nm or 157 nm and a linewidth less
than 1 pm.
[0312] The amplifier preferably comprises a discharge chamber
filled with a laser gas including molecular fluorine and a buffer
gas, electrodes connected to the same or a similar discharge
circuit, e.g., using an electrical delay circuit, for energizing
the molecular fluorine. The amplifier discharge is timed to be at
or near a maximum in discharge current when the pulse from the
oscillator reaches the amplifier discharge chamber.
[0313] The line-narrowing optics preferably include one or more
etalons tuned for maximum transmissivity of a selected portion of
the spectral distribution of the beam, and for relatively low
transmissivity of outer portions of the spectral distribution of
the beam. A prism beam expander is preferably provided before the
etalons for expanding the beam incident on the etalon or etalons.
Two etalons may be used and tuned such that only a single
interference order is selected.
[0314] The line-narrowing optics may further include a grating for
selecting a single interference order of the etalon or etalons
corresponding to the selected portion of the spectral distribution
of the beam. The resonator further preferably includes an aperture
within the resonator, and particularly between the discharge
chamber and the beam expander. A second aperture may be provided on
the other side of the discharge chamber.
[0315] The line-narrowing optics may include no etalon. For
example, the line optics may instead include only a beam expander
and a diffraction grating. The beam expander preferably includes
two, three or four VUV transparent prisms before the grating. The
grating preferably has a highly reflective surface for serving as a
resonator reflector in addition to its role of dispersing the
beam.
[0316] The line-narrowing optics may include an etalon output
coupler tuned for maximum reflectivity of a selected portion of the
spectral distribution of the beam, and for relatively low
reflectivity of outer portions of the spectral distribution of the
beam. This system would also include optics such as a grating,
dispersive prism or etalon, preferably following a beam expander,
for selecting a single interference order of the etalon output
coupler. The resonator would preferably have one or more apertures
for reducing stray light and divergence within the resonator.
[0317] In any of above configurations including a grating, a highly
reflective mirror may be disposed after the grating such that the
grating and HR mirror form a Littman configuration. Alternatively,
the grating may serve to retroreflect as well as to disperse the
beam in a Littrow configuration. A transmission grating or grism
may also be used.
[0318] The buffer gas preferably includes neon and/or helium for
pressurizing the gas mixture sufficiently to increase the output
energy for a given input energy and to increase the energy
stability, gas and tube lifetime, and/or pulse duration. The laser
system further preferably includes a gas supply system for
transferring molecular fluorine into discharge chamber and thereby
replenishing the molecular fluorine, therein, and a processor
cooperating with the gas supply system to control the molecular
fluorine concentration within the discharge chamber to maintain the
molecular fluorine concentration within a predetermined range of
optimum performance of the laser.
[0319] The laser system may also include a spectral filter between
the oscillator and the amplifier for further narrowing the
linewidth of the output beam of the oscillator. The spectral filter
may include an etalon or etalons following a beam expander.
Alternatively, the spectral filter may include a grating for
dispersing and narrowing the beam. In the grating embodiment, the
spectral filter may include a lens focusing the beam through a slit
and onto a collimating optic prior to impinging upon the beam
expander-grating combination.
[0320] A detailed discussion of the line-narrowing configurations
of an oscillator element of the laser system according to the
preferred embodiment is now set forth with reference to FIGS.
30a-30f. Several embodiments of an oscillator of the laser system
using line-narrowing techniques for the molecular fluorine laser,
are shown in FIGS. 30a-30f to meet or substantially meet the first
object of the invention.
[0321] FIG. 30a schematically shows an oscillator of a laser system
according to a first embodiment including a discharge chamber 202
preferably containing molecular fluorine and a buffer gas of neon,
helium or a combination thereof (see the Ser. No. 09/317,526
application), and having a pair of main discharge electrodes 203
(not shown) and a preionization arrangement (also not shown)
therein. The system shown in FIG. 30a also includes a prism beam
expander 230 and a diffraction grating 232 arranged in a Littrow
configuration. The beam expander 230 may include one or more prisms
and preferably includes several prisms. The beam expander serves to
reduce divergence of the beam incident onto the grating, thus
improving wavelength resolution of the wavelength selector. The
grating is preferably a high blaze angle echelle grating (see the
Ser. No. 09/712,367 application incorporated by reference
above).
[0322] The system shown includes a pair of apertures 234 in the
resonator which reject stray light and reduce broadband background,
and can also serve to reduce the linewidth of the beam by lowering
the acceptance angle of the resonator. Alternatively, one aperture
234 on either side of the chamber 202 may be included, or no
apertures 234 may be included. Exemplary apertures 234 are set
forth at U.S. Pat. No. 5,161,238, which is assigned to the same
assignee and is hereby incorporated by reference (see also the Ser.
No. 09/130,277 application incorporated by reference above).
[0323] The system of FIG. 30a also includes a partially reflecting
output coupling mirror 236. The outcoupling mirror 236 may be
replaced with a highly reflective mirror, and the beam may be
otherwise output coupled such as by using a polarization reflector
or other optical surface within the resonator such as a surface of
a prism, window or beam-splitter (see, e.g., U.S. Pat. No.
5,150,370, incorporated by reference above).
[0324] The system shown at FIG. 30b includes the chamber 202, the
apertures 234, the partially reflecting output coupling mirror 236
and beam expander 230 described above with respect to FIG. 30a. The
system of FIG. 30b also includes a diffraction grating 238 and a
highly reflective mirror 240. The grating 238 preferably differs
from the grating 232 of FIG. 30a either in its orientation with
respect to the beam, or its configuration such as its blaze angle,
etc., or both. The laser beam is incident onto the grating 238 at
an angle closer to 90.degree. than for the grating 232. The
incidence angle is, in fact, preferably very close to 90.degree..
This is arrangement is referred to here as the Littman
configuration. The Littman configuration increases the wavelength
dispersion of the grating 238. After passing through or reflecting
from the diffraction grating 238, the diffracted beam is reflected
by the highly reflective mirror 240. The tuning of the wavelength
is preferably achieved by tilting the highly reflective mirror 240.
As mentioned above with respect to the exemplary arrangement,
tuning may be achieved otherwise by rotating another optic or by
pressure tuning one or more optics, or otherwise as may be
understood by one skilled in the art.
[0325] FIG. 30c schematically shows another embodiment of an
oscillator having a laser chamber 102, apertures 234, outcoupler
236, beam expander 230 and Littrow diffraction grating 232,
preferably as described above. In addition, the system of FIG. 30c
includes one or more etalons 242, e.g, two etalons are shown, which
provide high-resolution line narrowing, while the grating 232
serves to select a single interference order of the etalons 242.
The etalon or etalons 242 may be placed in various positions in the
resonator, i.e., other than as shown. For example, a prism or
prisms of the beam expander 230 may be positioned between an etalon
or etalons 242 and the grating. An etalon 242 may be used as an
output coupler, as will be described in more detail below with
reference to FIGS. 30e-30f. The arrangement of FIG. 30c (as well as
FIG. 30d below) including an etalon or etalons 242 may be varied as
described at any of U.S. patent application Ser. Nos. 09/694,246,
09/771,366, or 09/686,483, each of which is assigned to the same
assignee and is hereby incorporated by reference.
[0326] FIG. 30d shows another embodiment of the laser system having
one or more etalons 243, e.g., two etalons 243 are shown. The
system of FIG. 30d is the same as that of FIG. 30c except that the
grating 232 is replaced with a highly reflective mirror, and the
etalons 243 are differently configured owing to the omission of the
grating 232 which is not available, as in the system of FIG. 30c,
to select a single interference order of the etalons 243. The free
spectral ranges of etalons 243 are instead adjusted in such a way
that one of the etalons 243, preferably the first etalon 243 after
the beam expander 230, selects a single order of the other etalon
243, e.g., the second etalon 243. The second etalon 243 of the
preferred arrangement is, therefore, allowed to have a smaller free
spectral range and higher wavelength resolution. Some further
alternative variations of the etalons 243 of the system of FIG. 30d
may be used as set forth in U.S. Pat. No. 4,856,018, which is
hereby incorporated by reference.
[0327] FIGS. 30e and 30f schematically show embodiments similar to
the arrangements described above with reference to FIGS. 30a and
30b, respectively, which differ in that the partially reflecting
outcoupler mirror 236 is replaced with a reflective etalon
outcoupler 246. The etalon outcoupler 246 is used in combination
with the grating 232 or 238 and beam expander 230 of FIGS. 30e and
30f, respectively, wherein the grating 232 or 238 selects a single
interference order of the etalon outcoupler 246. Alternatively, one
or more dispersive prisms or another etalon may be used in
combination with the etalon outcoupler 246 for selecting a single
interference order of the etalon 246. The grating 232 or 238
restricts wavelength range to a single interference order of the
outcoupler etalon 46. Variations of the systems of FIGS. 30e and
30f that may be used in combination with the systems set forth at
FIGS. 30e and/or 30f are set forth at the Ser. Nos. 09/317,527 and
09/715,803 applications, incorporated by reference above, and U.S.
Pat. Nos. 6,028,879, 3,609,586, 3,471,800, 3,546,622, 5,901,163,
5,856,991, 5,440,574, and 5,479,431, and H. Lengfellner, Generation
of tunable pulsed microwave radiation by nonlinear interaction of
Nd:YAG laser radiation in GaP crystals, Optics Letters, Vol. 12,
No. 3 (March 1987), S. Marcus, Cavity dumping and coupling
modulation of an etalon-coupled CO.sub.2 laser, J. Appl. Phys.,
Vol. 53, No. 9 (September 1982), and The physics and technology of
laser resonators, eds. D. R. Hall and P. E. Jackson, at p. 244,
each of which is hereby incorporated by reference.
[0328] In all of the above and below embodiments shown and
described with reference to FIGS. 30a-30f, the material used for
the prisms of the beam expanders 230, etalons 242, 243, 246 and
laser windows is preferably one that is highly transparent at
wavelengths below 200 nm, such as at the 157 nm output emission
wavelength of the molecular fluorine laser. The materials are also
capable of withstanding long-term exposure to ultraviolet light
with minimal degradation effects. Examples of such materials are
CaF.sub.2, MgF.sub.2, BaF, BaF.sub.2, LiF, LiF.sub.2, and
SrF.sub.2. Also, in all of the above embodiments of FIGS. 30a-30f,
many optical surfaces, particularly those of the prisms, preferably
have an anti-reflective coating on one or more optical surfaces, in
order to minimize reflection losses and prolong their lifetime.
[0329] Also, as mentioned in the general description above, the gas
composition for the F.sub.2 laser in the above configurations uses
either helium, neon, or a mixture of helium and neon as a buffer
gas. The concentration of fluorine in the buffer gas preferably
ranges from 0.003% to around 1.0%, and is preferably around 0.1%.
The addition of a trace amount of xenon, and/or argon, and/or
oxygen, and/or krypton and/or other gases may be used for
increasing the energy stability, burst control, or output energy of
the laser beam. The concentration of xenon, argon, oxygen, or
krypton in the mixture may range from 0.0001% to 0.1%. Some
alternative gas configurations including trace gas additives are
set forth at U.S. patent application Ser. Nos. 09/513,025 and
09/317,526, each of which is assigned to the same assignee and is
hereby incorporated by reference.
[0330] All of the oscillator configurations shown above at FIGS.
30a-30f may be advantageously used to produce a VUV beam 220 having
a wavelength of around 157 nm and a linewidth of around 1 pm or
less. Some of those configurations having an output linewidth of
less than 1 pm already meet the above first object of the invention
with respect to the linewidth. Those oscillators may be used with
other elements, such as an amplifier, as set forth below at FIGS.
31a-34b to achieve sufficient output power for substantial
throughput at a 157 nm lithography fab. Other oscillators producing
linewidths above 1 pm may be advantageously used in combination
with other line-narrowing elements such as a spectral filter, as
set forth below at FIGS. 31a-32b, and with an amplifier as set
forth in the embodiments of FIGS. 31a-32b.
[0331] FIG. 31a schematically illustrates, in block form, a laser
system in accord with a preferred embodiment, wherein a narrower
linewidth is desired than is output by the oscillator 248, and
higher power is desired than is output by the oscillator 248. To
reduce the linewidth, the output beam 220 of the oscillator 248 is
directed through a spectral filter 250. To increase the output
power, the beam 220 is directed through an amplifier 252.
[0332] The system of FIG. 31a includes a line-narrowed oscillator
248, a spectral filter 250 and an amplifier 252. Various preferred
configurations of the spectral filter 250 are described below with
reference to FIGS. 31b-31d. The oscillator 248 of FIG. 31a is an
electrical discharge molecular fluorine laser producing a spectral
linewidth of approximately 1 pm, and is preferably one of the
configurations described above with respect to FIGS. 30a-30f, or a
variation thereof as described above, or as may be understood as
being advantageous to one skilled in the art, such as may be found
in one or more of the reference incorporated by reference herein.
The oscillator 248 is followed by the spectral filter 250, which
transmits light in a narrower spectral range, i.e., less than the
linewidth of the output beam 220 from the oscillator or less than
around 1 pm. Lastly, the transmitted beam is amplified in an
amplifier 252 based on a separate discharge chamber to yield an
output beam 254. Preferably, the oscillator and amplifier
discharges are synchronized using a delay circuit and advantageous
solid-state pulser circuit such as is described at U.S. patent
application Ser. No. 09/858,147 and at U.S. Pat. No. 6,005,880,
each of which is assigned to the same assignee and is hereby
incorporated by reference.
[0333] The spectral filter 250 is preferably includes one of the
arrangements shown in FIGS. 31b-31d. Variations may be understood
as advantageous to one skilled in the art using any of a large
number of combinations of prisms, gratings, grisms, holographic
beam samplers, etalons, lenses, apertures, beam expanders,
collimating optics, etc., for narrowing the linewidth of the input
beam 220, preferably without consuming a substantial fraction of
the energy of the input beam 220.
[0334] FIG. 31b illustrates a first spectral filter 250 embodiment
including a beam expander followed by one or more etalons 258 to
yield an output beam having a linewidth substantially below the
linewidth, e.g., around 1 pm, of the input beam 220. Each etalon
258 includes two partially reflecting surfaces of reflectivity R,
separated by a preferably gas-filled gap of thickness D. The
transmission spectrum of the etalon T(.lamda.) is described by a
periodic function of the wavelength .lamda.:
T(.lamda.)=(1+(4F.sup.2/.pi..sup.2)sin(2.pi.nD
cos(.THETA.)/.lamda.)).sup.-1 (1) [0335] where n is the refractive
index of the material, preferably an inert gas, filling the etalon
258, .THETA. is the tilt angle of the etalon 258 with respect to
the beam, and F is the finesse of the etalon 258 which is defined
as: F=.pi.R.sup.1/2/(1-R) (2)
[0336] The reflectivity R and spacing of the etalon D can be
selected in such a way that only a single transmission maximum
overlaps with the emission spectrum of the broader-band oscillator
248. For instance, if the finesse of the etalon 258 is selected to
be 10, then the spectral width of the transmission maximum is
roughly 1/10 of the free spectral range (FSR) of the etalon 258.
Therefore, selecting a free spectral range of 1 pm will produce a
transmitted beam with spectral linewidth of 0.1 pm, without
sidebands since the linewidth of the oscillator (248) output
(approximately 1 pm) is significantly less than two times the
FSR.
[0337] Using multiple etalons 258 allows a higher contrast ratio,
which is defined as a ratio of the maximum transmission to the
transmission of the wavelength halfway between the maxima. This
contrast ratio for a single etalon is approximately equal to
(1+4F.sup.2/.pi..sup.2). Higher finesse values lead to higher
contrast. For several etalons 258, the total contrast ratio will be
(1+4F.sup.2/.pi..sup.2).sup.n where n is the number of etalons 258
used. Additionally, the spectral width of the transmission maxima
will be reduced with increased number of etalons 258 used.
Disadvantages of using several etalons 258 include high cost and
complexity of the apparatus and increased optical losses.
[0338] The beam expander 256 shown at FIG. 31b serves to reduce the
divergence of the beam incident onto the etalons 258. From the
formula (1), it follows that a change in the beam incidence angle
.THETA. causes a shift of the wavelength at which maximum
transmission occurs. Assuming an FSR of 1 pm, the etalon spacing is
D=1.2 cm. If the transmission interference spectrum of the etalon
258 is at its maximum at normal incidence (.THETA.=0), then the
angle .THETA., at which the transmission spectrum reaches maximum
again is .THETA..about.(.lamda./nD).sup.1/2=3.6 mrad. Therefore, it
is preferred that the spectral filter 250 shown at FIG. 31b be
configured such that the divergence of the beam is below .THETA.,
and preferably by a factor comparable to the finesse F of the
etalon 258. Since the divergence of a typical molecular fluorine
laser is several millirads, the advantage of using the beam
expander 256 to reduce this divergence from typically above .THETA.
as it is output from the oscillator 248 to below .THETA., is may be
understood. It is also preferred to use one or more apertures 234
in the oscillator 248 to reduce its output divergence (see the Ser.
No. 09/130,277 application, mentioned above).
[0339] The gaps between the plates of the etalons 258 are
preferably filled with an inert gas. Tuning of the transmitted
wavelength can be accomplished by changing the pressure of the gas
as described in the Ser. No. 09/317,527 application, mentioned
above. In addition to pressure tuning and rotation tuning of the
etalon's output transmission spectrum, the etalons 258 may be
piezoelectrically tuned such as to geometrically alter the gap
spacing.
[0340] FIG. 31c schematically illustrates a second embodiment of
the spectral filter 250 of FIG. 31a generally utilizing a
diffraction grating 260. Although there are other ways to configure
the spectral filter 250 according to the second embodiment using a
grating 260, an example is shown at FIG. 31c and described here.
The spectral filter 250 shown at FIG. 31c is a Czerny-Turner type
spectrometer, modified to achieve high resolution. The input beam
220 in focused by a lens 261a through an input slit 262a after
which the beam is incident on a collimating mirror 264. After
reflection from the mirror 264, the beam is incident on a beam
expander 266 and then onto the grating 260. The beam is dispersed
and reflected from the grating 260, after which the beam
retraverses the beam expander 266, and is reflected from the
collimating mirror 264 through an output slit 262b at or near the
focal point of a lens 262b. The output beam 259 then has a
linewidth substantially less than the linewidth, e.g., around 1 pm,
of the input beam 220, or substantially less than 1 pm.
[0341] The diffraction grating 260 is preferably a high blaze
echelle grating 260. The wavelength dispersion of this preferred
grating 260 is described by the following formula:
d.lamda./d.THETA.)=(2/.lamda.) tan .THETA. (3) [0342] where .THETA.
is the incidence angle. The spectral width .DELTA..lamda. of the
transmitted beam is determined by the dispersion d.lamda./d.THETA.
of the grating 260, the magnification factor M of the prism
expander 266, the focal length L of the collimating mirror 264 and
the width d of the slits 262a, 262b of the spectrometer:
.DELTA..lamda.=d(L M d.lamda./d.THETA.).sup.-1 (4) [0343] For
example, using an echelle grating 260 wherein the incidence angle
.THETA. is 78.6.degree., L=2 m and M=8, the slit width d which
would achieve 0.1 pm resolution for the spectral filter 250 of FIG.
31c is around d=0.1 mm. It is preferred, therefore, to reduce the
divergence of the oscillator 248 in order to increase the
transmission of the beam 220 through the input slit 261a. This can
be advantageously achieved by using apertures inside the resonator
of the oscillator 248 (see again the Ser. No. 09/130,277
application, mentioned above).
[0344] The third example of a spectral filter 250 that may be used
in illustrated at FIG. 31d. The spectral filter 250 of FIG. 31d
differs from that shown at FIG. 31c in that a collimating lens 268
is used in the embodiment of FIG. 31d, rather than a collimating
mirror 264, as is used in the embodiment of FIG. 31c. An advantage
of the embodiment of FIG. 31d is its simplicity and the absence of
astigmatism introduced by the mirror 264 of FIG. 31c at non-zero
incidence angle.
[0345] It is useful to reiterate here that synchronization of the
electrical discharge pulses in the chambers 202 of the oscillator
248 and amplifier 252 is preferred in order to ensure that the
line-narrowed optical pulse from the oscillator 248 arrives at the
chamber 202 of the amplifier 252 at the instance when the gain of
the amplifier 252 is at or near its maximum. Additionally, this
preferred synchronization timing should be reproducible from pulse
to pulse to provide high energy stability of the output pulses. The
preferred embodiment electronic circuitry allowing this precise
timing control is described at U.S. Pat. No. 6,005,880 and U.S.
patent application Ser. No. 09/858,147, as mentioned above.
[0346] FIG. 32a shows the use of a single discharge chamber 270
that provides the gain medium for both an oscillator and an
amplifier. The setup of FIG. 32a includes the discharge chamber 270
within a resonator including a highly reflective mirror 272 and a
partially reflecting outcoupling mirror 274. A pair of apertures
234 are also included, as described above, to match the divergence
of the resonator of this oscillator 248. A small portion of the
cross-section of the discharge volume is used to produce an
un-narrowed beam 276 with this oscillator configuration. It is also
possible to include one or more line-narrowing components with this
oscillator configuration, or to otherwise modify the oscillator
according to the description set forth above with respect to FIGS.
30a-30f.
[0347] Similar to the embodiment shown and described with respect
to FIG. 31a, this un-narrowed output is then directed through a
spectral filter 250, which is preferably one of the embodiments
described in FIGS. 31b-31d. Given the significant time (e.g.,
several nanoseconds) that it takes for the beam to traverse the
spectral filter 250, it is preferred to adjust the arrival time of
the filtered pulse to a second maximum of the discharge current. To
achieve this temporal adjustment, an optical delay line is
preferably inserted after the spectral filter 250. The delay line
may be one of those described at U.S. patent application Ser. No.
09/550,558, which is assigned to the same assignee and is hereby
incorporated by reference.
[0348] FIGS. 32b(i)-(iii) illustrate the electrical current through
the discharge gap, the intensity of the un-narrowed beam 276 and
the output 259 of the oscillator-amplifier system, each as a
function of time. The current exhibits several cycles of
oscillations, as shown in FIG. 32b(i). The optical pulse shown at
FIG. 32b(ii) evolves towards the end of the first maximum (a) of
current. The second maximum of electrical current is separated from
the first one by approximately 20 nanoseconds, thus providing
sufficient time for the beam 276 to traverse the spectral filter
250 and additional optical delay line 278. This discussion with
respect to the timing of the successive maxima in the electrical
discharge current reveals how the additional optical delay line 278
may be advantageously used to precisely tune the arrival time of
the pulse at the chamber 270 (amplifier). The line-narrowed beam
from the spectral filter 250, whose temporal pulse shape is shown
at FIG. 32b(iii), thus overlaps the second maximum b of the
electrical current shown at FIG. 32b(i) of the amplifier and is
amplified, and thus a line-narrowed beam 259, i.e., substantially
less than 1 pm, is output with sufficient power.
[0349] FIG. 33a shows the use of a line-narrowed oscillator
followed by a power amplifier made in a separate discharge chamber.
Any of the embodiments shown and described above including those
discussed with respect to the exemplary embodiments, the patents
and publications incorporated by reference, and the embodiments
described with respect to FIGS. 30a-30f can be used to narrow the
bandwidth of the oscillator. Examples of the preferred
line-narrowed oscillator 248 are set forth at FIGS. 33b-33f.
[0350] The line-narrowed oscillator 248 schematically shown at FIG.
33(b) uses a prism beam expander 320 and grating 232, preferably as
described in one or the U.S. Pat. No. 5,559,816, 298 22 090.3 DE,
U.S. Pat. Nos. 4,985,898: 5,150,370, and 5,852,627 patents, each
being incorporated by reference above. Alternatively, the Littman
configuration may be used (see discussion above with respect to
FIG. 30b). As discussed above with respect to the embodiments of
FIGS. 30a-32a, the additional apertures 234 in the resonator reduce
divergence of the beam and, therefore, advantageously increase the
resolution of the wavelength selector (again, see the Ser. No.
09/130,277 application for details).
[0351] The embodiment shown in FIG. 33c utilizes multiple etalons
243 as wavelength selective elements (see FIG. 30d). The prism beam
expander 230 in combination with the apertures 234 helps to reduce
the divergence of the beam in the etalons 243 thus improving
resolution of the wavelength selector. Additionally, this reduces
the intensity of the beam at a particular area of the surfaces of
the etalons 243, thus extending their lifetime.
[0352] FIGS. 33d-33e show alternative arrangements that each
include an RF or microwave excited waveguide laser as an
oscillator. The arrangement of FIG. 33d preferably includes a pair
of RF-electrodes 280 and a waveguide 282 preferably including a
ceramic capillary filled with a laser active gas mixture. Any of
the resonator configurations shown in FIGS. 30a-33c may be used in
this embodiment, wherein the discharge chamber 202 is replaced with
the RF-excited waveguide arrangement shown in FIG. 33d. Features of
the waveguide laser that may be used in the arrangement of FIGS.
33d-33e may be found at C. P. Christenson, Compact Self-Contained
ArF Laser, Performing Organization Report Number AFOSR IR 95-0370;
T. Ishihara and S. C. Lin, Theoretical Modeling of Microwave-Pumped
High-Pressure Gas Lasers, Appl. Phys. B 48, 315-326 (1989); and
Ohmi, Tadahiro and Tanaka, Nobuyoshi, Excimer Laser Oscillation
Apparatus and Method, Excimer Laser Exposure Apparatus, and Laser
Tube, European Patent Application EP 0 820 132 A2, each of which is
hereby incorporated by reference. RF-excited lasers are commonly
operated with a carbon dioxide gas medium, e.g., as discussed in
Kurt Bondelie "Sealed carbon dioxide lasers achieve new power
levels", Laser Focus World, August 1996, pages 95-100, which is
hereby incorporated by reference.
[0353] The specific arrangement shown in FIG. 33d includes a prism
beam expander 230 and a grating 232 in Littrow configuration. A
Littman configuration may be used here (see FIGS. 30b and 30f)
including the grating 238 and HR mirror 240. A pair of apertures
234 are again included, particularly for matching the divergence of
the resonator. A partially reflecting mirror 236 outcouples the
beam 220. An etalon outcoupler 246 may be used instead of the
mirror 236 (see FIGS. 30e-30f)
[0354] The arrangement schematically shown at FIG. 33e is the same
as that of FIG. 33d, except that the grating is replaced with a one
or more etalons 243 and an HR mirror 244. A grating 232 or 238 may
be used along with the etalons 243, and an etalon outcoupler 246
may be used instead of the partially reflecting mirror 236.
[0355] An advantage of this RF-excited waveguide type of laser is
its long pulse, which allows more efficient line narrowing, since
the linewidth is approximately inversely proportional to the number
of round trips of the beam in the resonator. Additionally, the
RF-excited waveguide laser has a small discharge width (on the
order of 0.5 mm) which allows high angular resolution of the
wavelength selector based on the prisms of the beam expander 230
and the diffraction grating 232. This holds for both of the
embodiments shown at FIGS. 33d-33e.
[0356] FIG. 33f schematically shows another source of a narrow
linewidth beam that may be used in accordance with the present
invention to serve as the oscillator 248 in the embodiment of FIG.
33a. The arrangement of FIG. 33f includes a solid state laser 285
with a third harmonic output at 355 nm, such as diode pumped,
Nd:YAG laser or other such type laser as may be described, e.g., at
U.S. Pat. No. 6,002,697, which is assigned to the same assignee and
is hereby incorporated by reference, or as may be otherwise known
to one skilled in the art. The solid state laser 285, in turn,
pumps a narrow linewidth tunable laser 286, such as a dye laser or
optical parametric oscillator, emitting, e.g., around 472.9 mm.
This 472.9 nm radiation is focused into a gas cell 88 containing a
mixture of halide metal and inert gas, in order to produce a third
harmonic beam at 157.6 nm. Such third harmonic generation in gases
has been described at: Kung A. H., Young J. F., Bjorklung G. C.,
Harris S. E., Physical Review Letters, v.29, Page 985 (1972); and
Kung A. H., Young J. F., Harris S. E, Applied Physics Letters, v.22
page 301 (1973), each of which is hereby incorporated by
reference.
[0357] FIGS. 34a and 34b schematically illustrate further
embodiments wherein a portion of the discharge volume of a
discharge chamber 202 is used as an oscillator with line narrowing,
and the same discharge chamber 202 is used as an amplifier 252. The
arrangement of FIG. 34a is similar to that shown at FIG. 32a except
that the linewidth of the beam 230 is narrowed within the resonator
of the oscillator, and no spectral filter 250 is preferably used. A
spectral filter 250 may alternatively be used in addition to the
line-narrowing optics of the oscillator of FIG. 34a. Again, the
line-narrowing arrangement of the oscillator may be modified as set
forth in any of the descriptions above (see particularly FIGS.
30a-30f, 33c and 33f), or as set forth in any of the patents,
patent applications or publications incorporated by reference in
this application, or as otherwise understood by one skilled in the
art, to produce a narrow output beam 220. The output beam 220 from
the oscillator is expanded by an external beam expander 290,
preferably comprising one or more prisms and alternatively
comprising a lens arrangement.
[0358] The expanded beam 292 is then directed through a delay line
278 (see the '558 application) to synchronize the pulse with the
amplification maxima of the chamber 270, as described above. The
optical delay line 278 serves to fine tune the arrival time of the
optical pulse to the amplifier section, similar to the embodiment
shown and described with respect to FIGS. 32a-32b(iii). The
expanded beam 220 then advantageously fills a substantial portion
of the rest of the discharge cross section, and is amplified.
[0359] In the above embodiments, it is preferred to adjust the gas
mixture in the discharge chamber 202, 270 of the oscillator, to
obtain the longest possible pulse. Additionally, the waveform of
the discharge current can be modified by deliberately introducing
an impedance mismatch of the pulse forming circuitry and discharge
gap. The impedance mismatch leads to a longer discharge time and
thus, to a longer optical pulse. The lower gain resulting from such
modification means lower efficiency of the oscillator. However, in
the embodiments discussed above, the amount of reduction in the
output power of the oscillator is regained at the amplification
stage.
[0360] An excimer or F.sub.2 laser in accord with a preferred
embodiment includes a laser tube filled with a laser gas mixture
and having a plurality of electrodes connected with a power supply
circuit for energizing the gas mixture. A laser resonator including
the tube for generating a sub-250 nm laser beam includes a line
selection unit for selecting one of multiple closely-spaced
characteristic emission lines around 157 nm, 193 nm or 248 nm.
[0361] In a first aspect, line selection is provided by a
transmission diffraction grating. The preferred grating is made of
CaF.sub.2 and also serves to outcouple the laser beam. The
transmission grating in accord with the first aspect advantageously
permits a straight, shortened and more efficient laser
resonator.
[0362] In a second aspect, line selection is provided for a laser
by a grism. The preferred grism also serves either to outcouple the
beam or as a highly reflective resonator reflector. The grism in
accord with the second aspect advantageously provides enhanced
dispersion and efficiency.
[0363] Also in accord with this aspect, an excimer or molecular
fluorine laser includes a laser output coupler including a grism,
which is a combination of a prism and a grating. Such a combination
of prism and grating within one element advantageously improves the
resolving power of a single dispersive element and reduces the
internal resonator losses by a minimum of optical interfaces.
[0364] The grism used directly as an output coupler for an excimer
or molecular fluorine laser advantageously combines four different
tasks in one element: partial reflection (or output coupling) for
the resonator, dispersion and line narrowing or line selection,
suppression of background radiation, such as amplified spontaneous
emission (ASE) radiation (or a parasitic second line), and pointing
stabilization of the selected line (wavelength). The grating-prism
(or grism) may be designed in such a manner that it realizes a
straight beam path for the selected wavelength which is used as the
output beam for use with an application process, operating like a
common mirror if the blaze angle of the grating is equal to the
prism angle for the selected wavelength. Thus, the laser resonator
can be very short even if it is containing a line selecting or line
narrowing element having the direction of beam propagation in a
straight line.
[0365] In a third aspect, line selection for a laser is fully
performed at the front optics module of the laser resonator
resulting in a more efficient resonator.
[0366] In a fourth aspect, a monitor grating and an array detector
are provided for monitoring and controlling the intensity of the
selected (and/or unselected) lines and for monitoring the stability
of the selected wavelength. The quality of the line selection may
be advantageously monitored.
[0367] In a fifth aspect, a laser system includes an energy
detector provided in an enclosure purged with an inert gas at a
slight, regulated overpressure. Advantages include reduced
turbulence typically associated with high gas flow and a reduced
rate of deposition of contaminants on optical surfaces. The fifth
aspect may be advantageously combined with the fourth aspect.
[0368] In a sixth aspect, a blue or green reference beam (e.g.,
having a wavelength between 400 nm and 600 nm) is used for laser
beam, particularly F2 laser) wavelength calibration and/or
alignment stabilization. The blue or green reference beam
advantageously is not reflected out with the red emission of the
laser and is easily resolved from the red emission.
[0369] In a seventh aspect, the clearing ratio of the laser gas
flow through the discharge area of a laser is improved. The reduced
clearing time is provided by narrowing the discharge width using
improved laser electrodes and/or by increasing the gas flow rate
through the discharge while maintaining uniformity by using a more
aerodynamic discharge chamber. Laser operation at higher repetition
rates is advantageously permitted by the reduced clearing ratio in
accord with the seventh aspect.
[0370] In an eighth aspect, an excimer or molecular fluorine laser
is provided with a substantially polarized output beam. The
polarization is provided by a thin film polarizer, a double
reflection prism and/or Brewster windows. The polarization provided
by the eighth aspect is advantageously 98% or better.
[0371] With respect to all of the above eight aspects, U.S. patent
application Ser. Nos. 09/738,849 and 10/116,903 are hereby
incorporated by reference as setting forth particularly preferred
embodiments in accordance with those aspects.
[0372] FIG. 35 shows an energy detector 349 for use with an excimer
or molecular fluorine laser system in accord with a seventh
embodiment and the fifth aspect above. A beam splitter 350
redirects a beam portion 351 towards the energy detector 349,
allowing the main beam 320 to pass through. The detector 349 may be
a diode or photomultiplier detector, and may be a diamond detector
such as that set forth in U.S. patent application Ser. Nos.
09/512,417, which are assigned to the same assignee as the present
application and hereby incorporated by reference. The detector 349
is preferably particularly designed to be sensitive at 157 nm.
Optics for filtering the red emission of the laser may be included
such as a dispersive element, holographic beam splitter, dichroic
mirror(s), or red light filter before the detector, or otherwise as
set forth at U.S. patent application Ser. Nos. 09/598,552 and
09/712,877, assigned to the same assignee and hereby incorporated
by reference.
[0373] The detector 349 is advantageously enclosed in a sealed
enclosure 352. The sealed enclosure 352 is preferably sealably
connected with a beam path enclosure 353 that encloses the path of
the outcoupled main beam 320 and that is itself sealably connected
to the laser resonator such that the beam 320 is never exposed to
and absorbed by oxygen and water in ambient air (see U.S. patent
application Ser. No. 09/343,333, which is assigned to the same
assignee as the present application and is hereby incorporated by
reference). The entire resonator itself is also kept free of the
photoabsorbing species such as by using a pair of smaller
enclosures 355a and 355b between the laser tube 302 and the rear
and front optics modules 330 and 340, respectively.
[0374] Photoabsorbing species such as oxygen, hydrocarbons and
water are removed from the enclosure 352, such as by pumping them
out with a high vacuum pump, such as a turbo pump, or by pumping
for a long time with a rotary or mechanical (roughing) pump. The
pumping can be continued until high vacuum is reached. However,
preferably only a roughing pump (not shown) is used and a series of
pumping steps each followed by purging with inert gas are performed
more quickly and with better results, such as is described in the
'333 application relating to the beam path enclosure 353.
[0375] After the contaminants are removed, a low flow of inert gas
such as argon or helium continuously purges the sealed enclosure
while the laser is operating. The enclosure 352 and the enclosure
353 may be open to one another such that the same purging gas fills
both enclosures 352 and 353, or the enclosures 352 and 353 may be
separately maintained. The flow rate of the purging inert gas is
selected such that only a slight overpressure is maintained in the
enclosure 352. For example, 1-10 mbar overpressure is preferred,
and up to 200 mbar overpressure could be used. The flow rate may be
up to 200 liters/hour, and is preferably between ten and fifty
liters/hour. The flow rate and pressure in the enclosure are
precisely maintained using a pressure regulator, flow-control
valves and a pressure gauge.
[0376] Advantageously, the slight overpressure, precisely
maintained, of the low flow purge in accord with the fifth aspect
may prevent the strain on optical surfaces that a high flow, high
pressure purge or a vacuum would produce. Fluctuations of the
refractive index with pressure in the enclosure may also be reduced
in accord with this fifth aspect. Moreover, turbulences typically
observed with high flow purges are avoided, and the rate of
contamination deposition on optical surfaces is reduced according
to this fifth aspect.
[0377] An attenuator 354 is preferably positioned before the
detector 349 to control the intensity of the incoming light at the
sensitive detector 349. The attenuator preferably includes a mesh
filter. The attenuator 354 may include a coating on the detector
349 such as is set forth at U.S. patent application Ser. No.
09/172,805, which is assigned to the same assignee as the present
application and is hereby incorporated by reference.
[0378] FIGS. 36a-36c illustrate a discharge chamber for a F.sub.2
laser in accord with a preferred embodiment and the seventh aspect
above. As noted, it is desired to operate the F.sub.2 laser at high
repetition rates (e.g., more than 1 kHz, e.g., 2-4 kHz and above).
To achieve this, the clearing ratio, or the gas flow rate (v)
through the discharge area divided by the discharge width (d), or
v/d, has to be improved over that which was sufficient at lower
repetition rates (e.g., 600-1000 Hz). This is because preferably
substantially all of the gas within the discharge volume at the
time of a previous discharge moves out of the discharge volume and
is replaced by fresh gas prior to the next discharge.
[0379] So, for a F.sub.2 laser having a preferred repetition rate
of 2-4 kHz or more, the clearing ratio to achieve the just stated
object would be 2000=v/d, or a value twice as large as for a
F.sub.2 laser having a repetition rate of 1 kHz. Thus, either the
gas flow rate v may be increased (without enhanced turbulence) or
the discharge width d may be reduced to increase the clearing
ratio. Both of these are achieved in accord with a preferred
embodiment. Preferably, this preferred embodiment incorporates the
discharge chamber design and electrode configuration set forth at
U.S. Pat. No. 6,466,599, which is assigned to the same assignee as
the present application, and which are hereby incorporated by
reference. Some of the preferred details are set forth below and
shown in FIGS. 36a-36c, and alternative embodiments are described
in the '599 patent.
[0380] FIG. 36a illustrates the tenth embodiment relating to the
shape of the main discharge electrodes 368 and 370, and the design
of the discharge chamber 302 itself. The shapes of the discharge
electrodes 368 and 370 significantly effect characteristics of the
discharge area 372, including the discharge width d. Therefore, at
least one, and preferably both, of the electrodes 368 and 370
includes two regions. One of these regions, the center portion 374,
substantially carries the discharge current and provides a uniform
and narrow gas discharge width. The other region, or base portion
376, preferably in collaboration with other conductive and
dielectric elements within the discharge chamber, creates preferred
electrical field conditions in and around the discharge area 372
and also contributes to the smoothness and uniformity of the gas
flow in the vicinity of the discharge electrodes 368 and 370.
[0381] The center portions 374 and base portions 376 preferably
form electrode 368 and 370 each having a single unit construction,
and composed of a single material. The center and base portions 374
and 376 may also comprise different materials, but the different
materials should have compatible mechanical and thermal properties
such that mechanical stability and electrical conductivity
therebetween is sufficiently maintained. The center portion 374 and
the base portion 376 come together at a discontinuity or
irregularity in the shape of the electrodes 368 and 370. A
significant deviation of the electrical field occurs at the
location of the irregularity in such a way that gas discharge
occurs substantially from/to the center portions 374 drastically
reducing the discharge width.
[0382] The center portions 374 are shaped to provide a uniform gas
discharge having a narrow width. The base portions 376 may be
shaped according to any of a variety of smooth curves or a
combination of several smooth curves including those described by
circular, elliptical, parabolic, or hyperbolic functions. The
curvatures of the base portions 376 may be the same or different,
and have the same direction of curvature with respect to the
discharge area 372, i.e., the base portions 376 each curve away
from the discharge area 372 away from the center portion 374.
Alternatively, the base portion 376 of the high voltage main
electrode 370 may have opposite curvature to the base portion 376
of the electrode 368. That is, the base portion 376 of the
electrode 370 may curve toward the discharge area 372, while the
base portion 376 of the electrode 368 curves away from the
discharge area 360. The alternative configuration provides an even
more aerodynamic channel for gas flow through the discharge area
372 because the electrode shapes both conform with the shape of the
gas flow.
[0383] The electrodes 368 and 370 may alternatively have a regular
shape and no discontinuity between base and center portions 374 and
376. The shape of the center portions 374 of the electrodes 368 and
370 in this alternative configuration is preferably similar to that
described above and shown. However, the base portions 376 taper to
the center portions in a triangular shape where the apexes of the
triangular shaped based portions 376 are the center portions and
are rounded as described above.
[0384] FIG. 36a also shows a pair of preferred spoilers 380 in
accord with a preferred embodiment. The spoilers 380 are preferably
integrated with the chamber at the dielectric insulators 382 on
either side of the discharge area 372. The spoilers 380 may be
integrated parts of a single unit, single material dielectric
assembly with the insulators 382, or they may comprise different
materials suited each to their particular functions. That is, the
spoilers 380 and the dielectric insulators 382 may be formed
together of a common material such as ceramic to provide an
aerodynamic laser chamber 302 for improved gas flow uniformity.
Alternatively, the spoilers 380 may be attached to the insulating
members 382.
[0385] The spoilers 380 are aerodynamically shaped and positioned
for uniform gas flow as the gas flows through the chamber 302 from
the gas flow vessel 384 (partially shown), through the discharge
area 372 and back into the gas flow vessel 384. Preferably, the
spoilers 380 are symmetric in accord with a symmetric discharge
chamber design.
[0386] One end 386 of each of the spoilers 380 is preferably
positioned to shield a preionization unit 388 from the main
electrode 368, and is shown in FIG. 36a extending underneath one of
the pre-ionization units 388 between the preionization unit 388 and
the main electrode 368. These ends 86 of the spoilers 380 are
preferably positioned close to the preionization units 388. For
example, the ends 386 may be just a few millimeters from the
preionization units 388. By shielding the preionization units 388
from the main electrode 368, arcing or dielectric breakdown between
the preionization units 388 and the main electrode 368 is
prevented. The spoilers 380 serve to remove gas turbulence zones
present in conventional discharge unit electrode chambers which
occur due to the sharp curvature of the gas flow in the vicinity of
the preionization units 388 and of the grounded discharge electrode
368.
[0387] FIGS. 36b-36c illustrate another feature in accord with the
seventh aspect above. As discussed above, the dielectric insulators
382 of the electrode chamber isolate the high voltage main
electrode 370. The gas flow is crossed by a first rib configuration
392a where the gas flow enters the electrode chamber 302 from the
gas flow vessel 384 and by a second rib configuration 392b where
the gas flow exits the electrode chamber 302 and returns the gas
back into the gas flow vessel 384. The ribs 394a, 394b, which are
current return bars, are separated by openings for the laser gas to
flow into and out of the electrode chamber 302 from/to the gas flow
vessel 384. The ribs 394a, 394b are preferably rigid and
conducting, and are connected to the grounded main discharge
electrode 368 to provide a low inductivity current return path. The
conducting ribs 394a of the rib configuration 392a are preferably
substantially shaped as shown in FIG. 36b. The conducting ribs 394b
of the rib configuration 392b are preferably substantially shaped
as shown in FIG. 36c. The ribs 394a and 394b of the rib
configurations 392a and 392b, respectively, are asymmetrically
shaped.
[0388] FIG. 36b is a cross sectional view A-A of the rib
configuration 392a through which the laser gas enters the electrode
chamber 302 from the gas flow vessel 384. The ribs 394a of the rib
configuration 392a each have a wide upstream end which meets the
laser gas as it flows from the gas flow vessel 384, and a narrow
downstream end past which the laser gas flows as it enters the
discharge chamber. Preferably, the ribs 394a are smoothly tapered,
e.g., like an airplane wing, from the wide, upstream end to the
narrow, downstream end to improve gas flow past the rib
configuration 392a.
[0389] FIG. 36c is a cross sectional view of the rib configuration
392b through which the laser gas exits the electrode chamber 302
and flows back into the gas flow vessel 384. The ribs 394b of the
rib configuration 392b each have a wide upstream end which meets
the laser gas as it flows from the electrode chamber 302, and a
narrow downstream end past which the laser gas flows as it enters
the gas flow vessel 384. Preferably, the ribs 394b are smoothly
tapered, e.g., like an airplane wing, from the wide, upstream end
to the narrow, downstream end to improve gas flow past the rib
configuration 392b.
[0390] The aerodynamic ribs 394a and 394b each provide a reduced
aerodynamic resistance to the flowing gas from that provided by
conventional rectangular ribs. Together, the effect aerodynamic
spoilers 80 and the aerodynamic ribs 394a and 394b permit the flow
rate of the gas through the chamber 302 to be increased without
excessive turbulence. The increased gas flow rate through the
discharge area 372, together with the reduced discharge width
provided by the advantageous design of the electrodes 368 and 370,
results in an increased clearing ratio in accord with high
repetition rates of operation of the excimer or F.sub.2 laser of
the preferred embodiment.
[0391] While exemplary drawings and specific embodiments of the
present invention have been described and illustrated, it is to be
understood that that the scope of the present invention is not to
be limited to the particular embodiments discussed. Thus, the
embodiments shall be regarded as illustrative rather than
restrictive, and it should be understood that variations may be
made in those embodiments by workers skilled in the arts without
departing from the scope of the present invention as set forth in
the claims that follow, and equivalents thereof.
[0392] In addition, in the method claims that follow, the
operations have been ordered in selected typographical sequences.
However, the sequences have been selected and so ordered for
typographical convenience and are not intended to imply any
particular order for performing the operations, except for those
claims wherein a particular ordering of steps is expressly set
forth or understood by one of ordinary skill in the art as being
necessary.
[0393] It should be recognized that a number of variations of the
above-identified embodiments will be obvious to one of ordinary
skill in the art in view of the foregoing description. Accordingly,
the invention is not to be limited by those specific embodiments
and methods of the present invention shown and described herein.
Rather, the scope of the invention is to be defined by the
following claims and their equivalents.
* * * * *